All men cheat

Unless you can see into the future, you have no clue what your DH will/will not do

Of course not all men will cheat, that’s a silly thing to say. I think people, generally, are foolish to say ‘so and so would NEVER cheat’ or, to a lesser extent, that they themselves would never cheat because you just never know, but to say all men cheat isn’t correct.

It's certainly common but no, of course not all men cheat.

I do think some people wrongly assume there can be no situations where they ever might, though. I've never cheated and don't plan to but if my husband were an arsehole, I was ground down and miserable and someone attractive offered me what looked like a lease of life...I don't know what I'd do, I've not been in that situation, but I can't be absolutely certain.

She's distrustful of men, it's her opinion. From experience. Let it go

Your friend is upset and disillusioned, which will be making her prone to believing the worst of men. But ultimately cheating is the same as anything that’s “forbidden.” Yes, plenty of people would cheat if they believed they’d never be caught - just as plenty of people would lie in a job interview, drive over the speed limit, steal, or commit fraud if they believed they’d never be caught. Cheating isn’t exceptionally different, it’s just another human behaviour. Obviously it isn’t all men, but plenty of men who appeared devoted family men and proclaimed to hate cheating have turned out to be unfaithful.

I don't agree with either of you.
She is wrong to say all men would cheat. Many wouldn't due to being very risk averse, having low sexual motivation, being too lazy to put the effort in, because they believe it to be wrong, or because they remain devoted to their primary partner and genuinely don't want anyone else till the end of their days.
You are wrong because you can't possibly know for certain if your husband will be in one of these categories for the rest of his days. No one knows another person's inner thoughts, no one can be certain peoples thoughts and feelings won't change. It's a long life and until we reach the end of it can never say with certainty what will or won't happen. Very few people who are betrayed didn't at one time believe their partner incapable of cheating.

Some men will, some won't. Mine certainly never would - neither my husband in over 50 years, nor our sons. I hate these generalisations.

It makes her feel better to think she can’t expect better because all men are the same, rather than facing the fact hers is a duff specimen. That’s sad for her and no reflection on your marriage.

She's hurting and spouting bollocks.
I'd find out what she wants from you, advice or just a shoulder or that you distract her with talk of other things.

And only engage if it's either of the latter two.