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 videos 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.