Should I ask her to stop flirting?

If he's fair game, he's fair game. She's allowed to do it; instead maybe you should flirt with him too and let the better woman win.

First think, she might see it as he's the guy she likes and doesn't know you do. If you tell her it could go one of two ways, she likes him too much and won't back down, or she wasn't that bothered and steps back. Or you don't tell her and instead make a move on him and either she likes him and now sees you as the intruder, or she doesn't and is fine. Morally it's a bit of a no-win situation, as to who ends up with him, unfortunately sidestepping her to talk to him is more likely to end up with it being you. No-one can tell you the right decision as we don't know anyone involved, but if you say nothing to anyone you will almost certainly not end up with him

Consider talking to her first. Try to understand her side and then have her understand yours. Then look at that guys point of view. He is pretty important in this as well. If he like you then good, go for it. If he likes her, then considering giving them space.