Most Helpful Guy
First off, you shouldn't be dating some guy who's forgetting things because he's been smoking weed... I'm sure you're better than that.
Anyways, there's no problem with talking to your ex ; you wouldn't want him being nosy if you were talking to an ex, would you? It's fine that maybe he's her friend, but you shouldn't slack off ; if he's cheating on you, you probably should know, so talk about his ex sometimes, or ask about her. Talk to his ex, too. Just, don't be paranoid... then he'll just think something's wrong/you don't like him anymore.