TEENS PLEASE HELP, TELL ME WHAT TO DO!?

Okay, so i have a friend, Krystene she is 16 and she is soo pretty and smart and just perfect! Well she has a boyfriend, Jaron, he is usually pretty cool but since i have been hangin out with them i have notice that Jaron is NUTS, he ALWAYS thinks Krystene is trying to flirt with other guys and constantly yells and downgrades her! When he is downgrading her she just sits there and ignores him, well yesterday when we were in the truck he grabbed her jaw really hard and then just pushed it back and he hit her and just treats her really bad. Well today at a party her cousin and a bunch of friends who all love Krystene dearly were all together and her cousin told us that she thinks Jaron hits Krystene, then asked me about it and said don't say anything to Krystene about it, well when she asked me i just said i don't know. My question is, should i tell Krystene that her cousin thinks Jaron hits her? Should i tell our friend who is close to all 3 of us? Should i say anything at all since Krystene's cousin said not to? Please tell me what i should do! And don't say that i need to tell someone that Jaron hits her, it's none of my business so i stay out of it.