My Friend’s Boyfriend

So my friend is dating this guy that honestly, tried to get with me first but I wasn’t interested, so he ended up trying with her and succeeded. Now that they are dating, me and him aren’t close friends but we’re friends. I started to realize that even around her he would be more touchy than normal but I would try not to say anything because I hate conflict and maybe I was looking into something that wasn’t that deep . But now he has gone the extent to flirting with me on Snapchat and one day he even called me asking me to f*** and I asked him about my friend and he said something a little disrespectful about her . So I thought maybe this is a joke but if I were in my friends shoes I wouldn’t find anything he said to be funny . How should I tell my friend without breaking her heart because she really likes this guy and she’s geeked over him. And honestly if he’s flirting with me knowing I’m her friend he has to be doing this with other girls . I can’t have my friend walking around stupid