If you want to be a good friend/ex gf than you should talk to him and mention liking his friend. Ask him if that's okay with him, but state that even if it's not, he can't really stop you from liking someone. Basically one of those "I'd like to hear your opinion but it doesn't matter if you don't approve" things. Then, of course, the answer would be yes, I think you should go out with a guy you like, without thinking too much about whose friend he is and stuff that will just ruin the date/possible relationship.
Good luck (:

If you really like him then go for it but you should talk to your ex first and ask him if its okay for you to go out with him if you really don't want to hurt his feelings but i think you should definitely go for it x