Would u keep mum or juz tell her?

My neighbour & I were dating for quite some time. However, things were not meant to be for us. Then, she got attached to a colleague of hers. What puts me in a spot is what I just found out about that particular guy. Apparently, I have a friend who is friends with this guy. Recently, I met up with him and I was shocked he knew about my brief relationship. It turns out that the guy was boasting to him about being able to snatch my neighbour from me. In addition, my friend stated that this guy as a heartbreaker and hyprocrite. He's the kind who will dump a girl after bedding her. Thus, my friend suggested I tell my neighbour to be cautious. I'm in a tight spot now. As a friend, I feel obliged to tell her. On the other hand, I'm afraid she might think I'm being childish & bad-mouthing that guy. What exactly should I do??