Jealous guys are wankers anyway. If you handle things in a mature manner, and make sure the girl sees that, then she'll probably leave the asshole and go out with you. Obviously, she's interested in some way because she's flirting with you.
Be as nice as possible to the guy, and make sure that by doing so he ends up looking like a giant walking asshole complete with turd crusties.

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Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi