She might be the problem in the marriage, forget it.
Years ago, I fell in love with my friend's girlfriend. When their relationship was falling apart, I thought this was the perfect opurtunity to take my friend's place. At the time, I thought their constant fighting and mistrust was his fault, I was so taken by her. Well, like the old saying goes, be careful what you wish for... I discovered it was the bitch who was largely at fault, and ended up having her empty my wallet, throw my life into turmoil, and nearly cost me a friendship. I suppose I had it comming.
Upgrade her to a friend with benefits. Get some while checking her out. Always can play the "don't want to lose you as a fried" card if she trys to progress the relationship.