Before you go any further you need to find out the truth. Her story maybe genuine or she could be telling a pack of lies. If it's the latter, stay well clear!
If it isn't I wouldn't feel guilty about homing in on some other guys chick if he was using her as his skivvy, and I wouldn't blame her for leaving him even though she's married. I don't think anyone should stay in an unhappy relationship. Life's too short. Worn that t-shirt and it's not worth it!