Why do girls flirt with guys they don't like?

one my my really close guy friends likes this one girl A LOT. they stared talking more and she keeps flirting with him, but she doesn't like him at all (only as a friend). when i told my guy friend that she keeps flirting, he doesn't understand why she is doing it if she doesn't like him. i can't explain it to him that that's just what girls do. so can you please help me and give me some sort of explanation that i can give him? thanks!