Should i go for the guy?

there's a guy i've liked for 10 years, but one or the other of us has always been in a relationship. recently we both broke up with our partners of about 6 months and there has been talk of us getting together as a couple, the only thing is his ex. she's not my friend but due to the guy being an ex (5 yrs ago) of my best mate who he has children with, his ex has become friends with her. she wants to get back together with the guy but he says no way, they were too different, different ideals etc. should i follow my heart and let things happen or stay away from him because this girl would be hurt to see him with someone else?