Most Helpful Guy
There are no "rules." Never think there are "rules."
People mention about "talking about the ex" being a bad sign because a lot of people who do, are not "over" their exes. By "over" though they can no longer be romantically interested but they haven't gotten over what has happened. This can be a foreshadowing of issues you may get stuck listening to or dealing with. If he only mentioned the ex from this time I wouldn't find it all that negative. But if he keeps bringing up hating his ex and stuff take it as a sign that he's not over her yet.
Most Helpful Girl
If you find that he keeps talking about her or their relationship, then he's not over her. If he just mentioned it that one time, he was probably saying it because he doesn't want anyone to hurt him again. Definitely monitor what he talks about because bringing up the ex on a regular basis is a red flag.