My best friend is a zoophilia?!?!?

Not too long ago, I went on my best friend's phone (with his consent) because I needed to delete an ugly photo of me. While I was on his phone, I went to his deleted folder and I found a video of his dog licking his penis and another video of him putting his penis inside the dog's mouth. I was totally shocked of what I just witnessed on his phone. I quickly closed the photo app and gave the phone back to him. To this day, he still does not know what I witnessed on his phone. What should I do?? ignore it or confront him about it? He's forcing the dog to do all these kinds of stuff. (BTW, we are 17, not sure if that matters)