If a girl is dating a guy and the guy is a catfish, what should the girl do to prove it?

The girl should first prevent getting scammed. She shouldn't be sharing any personal information about herself or anybody else. Then to try to prove, she can maybe ask to face time during the day, or asks him out to go somewhere (park, movies, etc.,) but make sure you take someone with you like a trusted adult or someone older, and if he keeps making excuses and denying then it's pretty clear it's a catfish.