Wow, that's low. Are あなた 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. または they could just not know what the problem with that is.
Your friend is a backstabber. Why would a friend 表示する embarrassing 動画 of あなた to your old boyfriend? She is betraying あなた and あなた should not be フレンズ with this girl. あなた can get better フレンズ than her.