I have kept something very hurtful from my friend am i a bad friend ?

Recently my friend got dumped by a guy i knew for years, he dumped her he said so he could "pursue his goals" she said okay she understood and didnt want to get in his way I recently found out that he actually had been texting to my other freind while they were dating and are talking aka dating now I feel like a horrrible friend for not saying anything my brother is close friends with the guy so he says don't say anything because it will lead to a bunch of drama and he blames it on the girl his buddy is talking to hesaid i shouldn't trust her because she initiated it.............But i believe she is the victim because of her situation, the guy is kind of a nice guy friend wise but he also is kind of a ladies man i think what he did is so low and now i've been in knots about this because i havent said anything to her and if and when she finds out she'll hate me for it because she is not over him i wouldn't blame her though she is a sweet person and i just don't want her to become bitter and resentful from this