personally, i wouldn't do it, there is boat loads of drama when you hook up with a girl who is just dating another guy, i couldn't imagine the drama that would be wrapped up in a girl who is married
but it looks like you're hungry and everyone needs to eat, so with that being said, if you're gonna have a fling with her, or any other girl who is involved with another man stick to these 2 basic rule of thumb and everything should be fine
1) can he kick your ass? if no, make sure he doesn't have close friends/relatives who can, most guys would bang her if potential danger #1 is clear, but it smart to check off #2 as well which is ...
2) is he of higher status than you? what i mean by that question is this, is he in a position where if he found out, he could make your life a living hell for shits and giggles, for example, if you're a drug dealer, i wouldn't have a flind with the town judge's wife .. it could come back to bite you in the ass for a long, LONG time (plus the guys in prison will make sure its a pain in the ass literally)
now if you get passed 1 & 2 with flying colors, you still have to be careful with the shit ton of drama that will come your way, because the husband will find out eventually and shit will most likely hit the fan, what you have to decide is this
is all that was mentioned above (and most likely a lot more) worth a piece of ass? again, personally, i think no, but some guys are willing to put up with it ..
be careful