Awesome French

Is it possible that French people say "tu es fou" when speaking to girls? I've heard someone French saying it to a girl (or I think I did haha) - but it should be "folle", non?

Yes, it’s possible (I even do it myself sometimes).  It’s sheer slang though.  The correct sentence would be “tu es folle” of course, but well..  It’s spoken French so..