@LetHimHaveIt
'What support does she need? If shagging someone else is upsetting her, she can stop cant she?'
Don't bother. I've asked that question two or three times, and none of the 'Not much of a friend, are you?' brigade can answer. For glaringly obvious reasons.
Wow. That's a bit strong!
Look, I'm as anti affairs as the next woman on here. They destroy families and lives. This I understand. And as far as I can see, the OP is not being asked to be involved in the affair, only considering whether she will support her friend.
In answer to the question, what support does she need, well that would depend wouldn't it. She may just need to know she still has a friend.
Here's a scenario for you. Friends DH is an arse. He is threatening to leave her homeless and penniless because of her actions. She is scared that she will lose everything and friends with it. Now, the few closest friends I have are like family to me. Even when my family totally fuck up, I still love them and I'm still there for them. Even when I completely disagree with their actions. I cut them some slack because I love them. I'm not a doormat. I'm not agreeing with them. I'm just still there for them, listening to them, offering advice when asked for, and practical help when required.
Contrary to Mumsnet opinion, people are not black and white. There are myriad shades of grey in between. Nobody is all good or all bad. You can be a good person doing shorty things or a bad person doing good things.
So yes, I would stand by my friend without condoning/approving/liking their actions. It would take a lot more than an affair for me to change that. If you were talking murder here or serious criminal activity then I would be reviewing those friendships. That's just where my personal bar is.
Op asked for advice. Asked what we would do. So that is what I would do.