I don't believe in holding back, period. If you want to be with someone, go for it. You never know what you're missiing out on when you pass up a chance. If your friend is a true friend, they will probably understand. The only thing you need to be careful about is getting hurt yourself. You should probably look at what happened between that person and your friend because there might be some incompatiability there between you and your friend's ex. If you feel good about it, jump on in. If your friend gets mad, that's his deal.