Is It Ok To Date The Ex Of A Friend??

i have a friend who came to me for advice. There is a female he really has an interest in...but she just so happens to be an ex of a close friend of his. He asked me if he would be wrong for going after her and expressing an interest in her. Lord knows i am terrible at relationship advice, which is probably why i am single lol...so i had NO clue what to tell him.

i know there is a "code" about not dating the ex of a friend but ummmm...what exactly is the code? Is there a certain closeness you have to have with the person before it applies because i know some dudes who share females...and not just for sex (they actually date the same girls)...dont ask me, im just the messenger lmao

but yeah...what the hell should i tell this dude? shit, i just want to know an answer for myself!!!