I wouldn’t like this if I’m honest. I might also have a bit of a backwards view on opposite sex friendships though.. I’m fine with pre established friendships, new work friendships, sports etc but not with messaging every day and being a shoulder to cry on for 18months, especially as it started online.
I’m not sure if it’s just the general attitude of how and where I’ve grown up, but the majority of my friends feel the same way- that there’s boundaries when you’re in a relationship. I’d class my friends husband as a close friend of 15 years and I don’t even have his phone number! I’ve gone through her if I’ve needed anything from him.
that’s not to say I don’t think men and women can have platonic friendships, I think it’s more than doable to keep them this way, but secretly messaging each day rings alarm bells for me. Maybe I’m a bit of a jealous person (and also biased from past experience) but I think messaging someone every day and hiding it from your partner is disrespectful - and that’s without cultivating an emotional relationship (helping her through a breakup sounds emotionally invested to me) and this is where feelings can get hurt, for you, and even your partner or this woman if one gets the wrong idea.
id want to see the messages, even if this made me look paranoid. I’d explain that I feel uncomfortable and would just like to see what sort of relationship this friendship looks like.
i suppose it depends on how your husband usually behaves - is he sociable and friendly to everyone and makes friends easily, helpful and supportive to them all? If this is his character and all is innocent, then fine.
if this is out of character, I’d have concerns.
i would also try and feel where your concern is coming from - is this coming from a place of anxiety/discomfort,,or do you have a gut feeling something is amiss?
and not to tar all with the same brush or jump to unwarranted conclusions - if you feel something is inappropriate, don’t let him sway you with the fact she’ll be coming as a friend and meeting you/family so there’s clearly nothing going on - after my ex, someone told me ‘they hide in plain sight’ and this really stuck with me.
ask to see the messages, if he’s reluctant, it could be no more than he’s had a bit of a whinge to her he’d rather you not see, or it’s inappropriate conversation.
as for the woman, I cannot imagine striking up a conversation online with a random man then coming to see him, his partner and his family having done no more than have friendly messages and never been mentioned to any of them. But this is just my way of thinking I guess!