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?

Unsolicited advice is always hard for parents to hear (especially since they get it so often). I would suggest treading carefully and strategically. You could wait until it's just the 2 of you in a relaxed setting. Then ask her about her motherhood journey so far. Questions like “What’s the most difficult part of motherhood?” will get her to open up and bring up her child’s negative behaviour. This is when you can give advice because she’ll really be listening for it as opposed to being surprised by it and feeling attacked.

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.