Stupidy..i need an advice!

So, when i got my baccalauret i mad a little party between friends.
There was this chic amiga of mine who had those amazing sunglasses. anyway, we danced we laughted we had fun! But in a specific time, i went out with my 5friends and my sister and her friend stayed in the house with my friend.
Sudently, my friend disappeard, they didn't know why she left that way and didn't even say goodbye. Sudently my friends sister realised that her sunglasses where gone. They worth like 400 dollars, she was so pissed she was mad, and she soon knew that my friend took them since she's the only one who was there exept my sister.
Anyway, in a few days later.. she met her, she took her bag .. saying that she won't give it back untill she gets her sunglasses back.
I was pissed because of this situation, i was with my ex's friend .. i told him what happend.
And because he's a childish stupid guy, he didn't stop calling my friend ( his ex) with a unknown number and saying mean things to here, he called me also and he was saying stupid things like " hello your friend sold me a sunglasses but i knew now that they were stollen, i wanna give them back to you "; the first time i didn't knew it was him but when i talked to my friend and she said that she receives calls from this guy too .. i knew right away that it was .. because he hate her, he wanna do everything to make her regret her life!
Now, i'm feeling guilty about this because it was me the one who told him about this problem.. Why can't i keep my moouth shut! the girl did nothing bad to me .. i feel so pissed of my self.
And i'm scared that one day he will tell her and she'll get mad at me!
i don't know if i should talk to him or let it go.
P.S : he's very mean guy and he's constatly offensiv to everyone! he gets mad really quickly; i'm scared if i talk to him he would go to her and say blablababla!
What should I do ?!