Well, first off, why do you let this friend know you don't like her boyfriend? You're not with him, so what do you care? And if she goes on about him and how he treats her wrong, LET HER KNOW it's not a topic of conversation you're going to engage in anymore. It's not that hard to do.
Who cares if he's violent and abusive? She's with him, she already knows how he is, but she's stayed with him anyway (probably because she has some dire issues which you can't fix).
Just leave it alone. It's not up to you to give her a report about her boyfriend's extra activities. And i'm sure she already suspects it anyway. It's none of your business, never will be. SHE is with him, again... and you're not.