What would you do??????

Hello everybody, I am a 44 year old mother of 3, and my youngest son, has had a best friend for about 7-8 years now, and I came home and my bathroom door was half way open. So next thing I saw my sons friend in there, and it startled me because their he was sniffing my thong and he found my toy under my sink and was looking at it while pleasuring himself.. I quickly went into the garage and waited 15 minutes and went inside and acted like I didn't see anything and so did he. He never knew I saw him. I went back into my bathroom later that evening and my thong was in the hamper full of semen. And toy was put back nicely where he found it. Do I tell his parents? Do I talk to him? Do I just let it go? What do I do?? He's a nice kid and always around for my son.  He's 19, that's too old to tell his parents? What do I do? I'm panicking.