Wow, that's low. Are you sure they're really best friend quality? They could be doing it for many reasons... Maybe they feel insecure. Maybe they have a crush on him. Maybe they want to look cool. Or they could just not know what the problem with that is.
Your friend is a backstabber. Why would a friend show embarrassing vídeos of YOU to your old boyfriend? She is betraying you and you should not be friends with this girl. You can get better friends than her.