I kissed my best friend's ex boyfriend. Was I wrong?

my best friend broke up with her boyfriend about a month ago, after about a 2 1/2 month relationship. He was grounded for about the last month and they didn't speak at all. She told me a few weeks ago that she's completely over him, and even likes someone new. I didn't meet her boyfriend until after they had already broken up, and a few days ago we made out and cuddled for a while. I told my best friend, and she was totally cool about it, and even told me she was happy for me. Later, however, she posted online that she's too trusting and it always f*cks her over in the end. Should I pursue a relationship with this guy? Or should I stay away, despite my best friend being "okay" with it?