I feel like I have F***ed up

We have exchanged nudes and stuff like that, anyway, I started to noticed him getting interested in my best girl friend (she is 15 and he is 19, my age) and confronted him about it, he said he wasn't interested in her for various reasons, one being the age difference. 

On NYE he asked her for nudes, which she said no to, he got quite annoyed and told her to F*** off, now I was abit pissed at that but I put it down to him being drunk as. But acouple days later he asked her if it was going to go anywhere between them and if she liked him, which pissed me off again. She said no she didn't like him but the next morning she messaged him saying she did she just didn't want to say it around me, but her feelings didn't come up until all of a sudden he seemed interested in her. 

I confronted him again and he asked why I cared so much and I told him that I liked him and have for a while and also said I didn't want her to grey hurt, which I can guarantee would happen, not because he would break her heart but because she gets attached and clingy very easily and if anything were to happen, she would have a meltdown if he didn't talk her for 10 mins (which she does to me all the time). He got really offended by that comment and wouldn't even let me explain what I meant and won't talk to me at all. It's killing me because I see him more then twice a week at work and I feel really bad but he won't even let me explain. I don't know what to do! Help please!