I'm sorry to be blunt but I would LEAVE his ass over this. Seriously, this has been going on for a year and he hasn't put a stop to it? Why? That's really, really suspicious, b/c I know if there were a situation like this in my life that my husband and family are more important than an ex, and that would have been explained to the ex in NO uncertain terms, just before I told them to piss off.
I expect the same respect from my husband.
Personally, I have enough stress in my life w/o having to nag my husband to tell his ex to piss off. Is he having an affair or not? Well, w/o him around, I wouldn't have to worry about that a/more.
As for her, well, after I bailed, I'd call her up and leave a line from an old Country WEstern song on her voicemail: 'If she wants a man who'll take the ring off of his hand, and then turn around and say that he'll be true. She deserves you.'