Depends on so many things.
Committed or single? I hate it when women become exclusive to men who aren't exclusive to them. Grinds my gears. Heifer you ain't cheating, you're dating!
Has HE cheated on her? If yes, welp, payback's a B. I would warn my friend though that two wrongs don't make a right and if they want to truly salvage their relationship, as much as it sucks, suck it up and work things out.
Either way, if a friend asks me to do this, she ain't a good friend. I'm not trying to get stabbed by your dude and a real friend wouldn't put me in a situation like that IMO.