Dating my Ex's Best Friend

Hello, I need some advice. My relationship with a guy ended about 1 1/2 year ago. He was the one to end it saying that he had too many issues going on (selling his home and a custody battle). I gave him space but let him know i was still interested. He kept telling me it wasn't a good time for him to date. Not even 2 months later he started dating someone else. It was gut wrenching to see him with someone else after he told me he had to put dating on hold. Any ways, recently his best friend asked me out and i was wondering if it is a good idea to go out with him. I like his friend alot(never thought about dating him)but him and i always have been friendly. Would it cause problems or should i not even worry about it since he broke up with me and moved on quickly?