This is a tricky topic and while many people have strong feelings against this (never! friends come first!), I think there are some gray areas to be considered. Ask yourself some questions and really think about what it would mean for you and for your friend. First of all, is your friend OK with it? How close a friend is he/she?
Do you feel strongly enough about this person to make it worth bring it up with your friend and risk him/her being angry at even being asked? Who ended the relationship? How long ago was it? How serious was the relationship and how long did it last? You will probably hear varying advice on this from different people, but I believe there is no right answer because every situation is different. The bottom line is, think about others but think about yourself, too. If this is a good friend of yours, be careful, you don’t want to lose a friend over a passing fling. On the other hand, if your friend is truly OK with it, this “fling” just might be worth it.