I think I would first ask about the nature of her relationship with her husband.
If she married him for his money and was out for some fun, then I'd tell her to get the hell out because I don't want to be her pleasure toy.
If she told me a story that basically amounts to once liking being married, but the relationship isn't what it used to be, then I'd ask her if she had tried communicating with her husband. After all, the reason most people have affairs is to make up for something they lack in their relationship. If she can't talk to her husband about whatever it is, maybe she should consider a divorce. Of course, all that discussion would be a complete mood-killer, so in regards to OT, I guess that's also a no.
If her husband is the type who goes away for long periods of time on business, I'd ask if she suspects he cheats on her while away. If she does and knows for certain, I'd say "Alright. But I'm not going to be an item of revenge here. We're doing this because you and your husband can have an understanding about this." If she merely suspects but doesn't have proof, I'd tell her that she can't use me to lash at him for something she's not even certain happened. If she knows for certain he doesn't cheat, then I'd say "And you're sure okay with doing this? Well, in that case..."