Wow, that's low. Are 你 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. 或者 they could just not know what the problem with that is.
Your friend is a backstabber. Why would a friend 显示 embarrassing 视频 of 你 to your old boyfriend? She is betraying 你 and 你 should not be 老友记 with this girl. 你 can get better 老友记 than her.