<human>: Can you summarize the given text in a few sentences?

Okay. Here's the context. Her boyfriend is a douchebag. I thought he had grown up and changed but seems like he didn't. Before they were dating, this guy strung along one of my friends with no intention of dating her, another friend liked him and even went down on him and he up and ditched her. The douchebaggy part is that he sits here and pretends he's this wonderful Catholic boy who talks about how to be a good Catholic man and how to act in relationships when he does this shit to people and that's probably the most frustrating thing to me - the hypocrisy. Anyways.

Okay, so this guy, who we'll henceforth name Douchenozzle (DN), has been dating my friend we'll call Sarah for a while. A little over a year I think. Recently, my roommate asked us if we knew him and I said yes. She then asked if he was dating someone, to which I replied yes. My roommate then tells me that DN was messaging her friend on Facebook, saying that he liked her and that he isn't dating anyone right now. Typical DN. Cue frustration. So I asked her what her friend was going to do and her friend ended up confronting him, but not telling Sarah. This girl didn't know her, so I can sort of see why she was hesitant.

Here's my question. How do I tell Sarah without it coming across like "a friend of a friend said this..." sort of sketchy sounding, right? I don't want her to keep being with a guy that does this to her, but I just am unsure how to approach it. If you guys could help me out, I'd appreciate it.
<bot>: Friend is dating a douchebag who tried to cheat on her. I heard about it secondhand and don't know how to tell her.