What to do if a girl likes you but your friend likes her?

People are not objects that can be claimed by somebody else. If she likes you and you are interested in her, pursue a relationship if you want to. Your friend can’t call dibs on a human being.

If it’s a situation where she just dumped him and is now looking toward you, I’d say give it a fair bit of time before going on a date with her, if at all.