No, NO, and HELL NO.

You never date a friend's ex. NEVER. If they are truly your friend, then you respect that, period.

Some of you have not been fully cooked yet, so this is where having been around the block a few times, i.e. being old, is advantageous.

Friend's exes should always be strictly off limits, UNLESS you either have, A: the friend's blessing, or B: it was just some menial one-night type of relationship. If they dated for any reasonable length of time or you KNOW that he/she (your friend) had feelings for that other person......then its absolutely "NO".