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.