My friend doesn't seem to really discipline her child, and when we go out, he can get a little bit rowdy and it's a little embarrassing. How can I bring up the subject of her son's behavior tactfully?

This is a bit of a tricky subject, but perhaps you could try suggesting when to reprimand her child in a very non-abrasive fashion. For example, if her child is loud or rowdy in a public place, you can suggest that your friend should do something because her daughter is the only one acting up. Or something like that. It just depends on the situation or severity of the behavior. Also, you need to come of not "judgey" in the process.