They hit on you to see if you like them. Not because they particularly want to get with you, but because they want to see if they can. He probably knows you won't do anything with him. She's your best friend. Never let a guy destroy your friendship.
Friends usually last longer than boyfriends.
Just tell the guy to stop it. My friend Michelle hangs out with Liz's boyfriend Greg. Liz is her best friend and lives in Washington. Whenever they're alone, Greg would slip his arm around Michelle and lean in a little too close. You don't need to go running to your friend.