To tell my friend that her husband is trying to cheat on her - with me?

What??? Oh behave.

Block him, the oh you’re such a joker feels more flirty than shutting it down.

Why are you dithering? Message now and say his earlier contact was deeply inappropriate and block the sleazy bastard. Then show his wife.

The vote is meaningless because OP has asked 2 contradictory questions within the same post. The title asks if she'd be unreasonable to tell her friend, but by the end of the post the question changes to is she unreasonable to keep quiet. I've voted based on the final question but others have probably voted according to the title.

written communication is somewhat different to being confronted face-to-face though @GretchenWienersHair

How about something like this

"Tom [sorry to all the Tom's]
I've considered and reread your messages and they are really inappropriate. I initially thought they were a bit jokey but somewhere along the way they have completely crossed the line. I am Mary's friend and to be quite clear, I have no interest in "having fun" with you if she is away. Feel free to free her up for a night so we can go out and catch up though.
I don't want to get any more messages like this or I will have no choice but to tell Mary and to show her a copy of the messages I have taken screenshots of. Assuming these messages stop abruptly, I will assume it was a momentary lapse of judgement which won't be repeated with me or any other of Mary's friends."

ThisLoudBeaker

I would block him completely and not see her when he's there.

Don't tell him you are turning him down because his wife is your friend.

Just turn him down flat. Tell him to fuck off if necessary.

Exactly
Why the hell let it go on weeks?
This won't end well for the op

Saying you didn't want to cause drama is a bit wet as a reason to not tell him to fuck off.
What drama would have been caused? Would he have told anyone? No.

When I was newly single...Jesus, the creeps came out.
One guy asked me to go to the cinema with him. I have absolutely no problem going out with a male friend, but if they are in a couple then their partner needs to be fine with it. Sometimes I know because their partner will be right there while we plan something.
To this guy I said "does Helen know you've asked me to the cinema?". I suspected not.
Got a load of "she doesn't understand me" bollocks.

I killed what I thought was a friendship off.

I would message him to ask that he does not send you any more messages without his wife's consent. She is your friend.

If he carries on, then don't tell her that he is trying to have an affair, but do ask her to tell him to stop messaging you as it makes you feel uncomfortable.

If you say he is trying to cheat on her with you there is a risk that she will find a way to put this on you as the femme fatale. You need to be clear that you are not a trouble causer, but you want this to stop.

Chances are high that he will attempt to turn it round on you if he thinks you might tell, and will portray you as chasing him and offering it to him on a plate.

I'd get in first.

You're likely to lose the friendship either way, so I'd prefer she got the truth than got his version first.

Hi, that's a hard one because you feel damed if u do and damed if you don't but just think if was you, you would want to know but put to him first piss off I don't like you and I love my mate you are taking the piss so I tell you what I'll pop around and we will show her the messages together? Because fella you are a piss taking wanker and I'm not having that for my mate she deserves better!! And then I will take her out on the town and get her a nice proppa man ...sorted 👍

Unfortunately your brush of "Haha you're such a joker" won't be taken as a brush off. You've basically just complimented him and encouraged further communication by not shutting him down.
I get why you tried to sound pleasant and hope that he would take the hint. But he didn't give a shit that he was putting you in this situation, so why should you consider his feelings in this.
I've found from experience you need to be direct with no chance of confusion.
"Please don't send me suggestive messages, they are inappropriate and I do not want them".

I think if you tell his wife now having done nothing in writing to shoe her you tried to shut it down, then she's going to think you're as bad. Which is very unfair, but time and again we see the woman made the scapegoat for men's behaviour, because its easier to believe badly of a friend and drop them from your life that to get rid of your husband and upend your life. I'd expect him to say he was joking, you took him seriously, even lie that you tried to take it further and are now a woman scorned or some such. I'd message him "Bernard, I'm uncomfortable with your messages to me. They are flirty and suggestive and not appropriate from my friends husband. I don't want any ill feeling because friend matters to me, but I doubt she'd be happy with this, and I'm certainly not. So I don't want you to message me again"

These “you’re enjoying it, aren’t you” comments are gross and the reason women don’t speak up when we’re put in uncomfortable positions (or worse) by men.

“Don’t make a scene”. “He’s just being friendly”. “Not everyone fancies you, you know” . “She’s just looking for attention”. “She was up for it”. . “Why didn’t you say ‘no’?” . “She was asking for it”.

Any of those sound familiar? Don’t feed into that.

These messages are very clear and you should have knocked it firmly on the head after the first one.

You are where you are though. He’s a sleaze and I would want to know if my husband was sending these sorts of messages.

You may upset the apple cart OP but you’ll have given her a real chance of making an informed choice re staying with such a sleaze. You never know, this might be just the push she needs to leave him (no one knows what goes on behind closed doors) or the beginning of her realising who he really is.

Don’t send screen shots, if you’re close. Go round for a brew and sit down with her face to face when he’s not there and take her through the messages.

If your reply’s to this mean you are not confident doing that then you need to think what to do ie screen shots and a word with yourself.

either way let her know.

incidentally what happened in the last few weeks to trigger this? He can’t have just text suggestive messages out of the blue?? Was there a party/dinner/lift home etc where you were talking etc?

I'd have told him to fuck right off after his first inappropriate message
You've let it go in weeks

so what are you going to do @ThisLoudBeaker?

Honestly, this needed to be shut down hard at the start, but better late than never. Screenshot it all and speak with her face to face, apologising for not having been faster to tell him to get lost and for not having told her sooner.

@ThisLoudBeaker Make it quite clear and say something like "Stop messaging me, I am not interested so go away and leave me alone!".

Don't say anything at all like 'how can you think I'd do that when your wife is my friend?' because all that will do is make him think that you do fancy him and the only thing stopping you is your friendship with his wife.

Shut him down completely.

not harsh, if a friend of my partner sent dubious messages to me, he'd definitely get short shrift from the outset!