'Sup with all the zombie threads recently? Did somebody break open that canister of Trioxin that was left in WP by mistake?
Anyway...generally not advisable. I'm married to my ex-fiancée's would-be bridesmaid. Dating a friend of an ex leaves your relationship open to unwanted negative influence. Unless you can pull off the creepy isolation thing (bad idea, don't do it), you can't keep your lover away from everyone who wants to badmouth you. When they do this, it's a bad sign. If your gf won't stand up to her friends for you, it's danger Will Robinson. Dating an ex's friend just INVITES this kind of thing.