Sorry Postdog, I answered on the other thread before I saw this one. What type of emotion involved? If you're talking about something that would mean more than just sex it might be better if she left the hubby and had a bit of breathing space before embarking on anything more serious.
If it's just sex, it does sound like it's well on the way to happening. Erm, it will change things in your friendship, no doubt about it, sex always does. But if you think you can handle it then I guess it's up to you two. You're both consenting adults.
Personally, if emotion may be involved, I wouldn't go near it with a barge pole as situation could get seriously confusing. But that's just me.
Good luck and I hope it works out for you whatever you decide to do xx