What should a visiting family member do at an aa meeting for a loved one?

Listen, learn & Accept. Be as respectful as you would be if you were visiting someone's home. Take care.
Be reserved. It's good that your loved one is there and that you have been invited to an open meeting. This is one of these times when simply being polite and saying, "thank you" to the aa folks, especially the sponsor if there is one, is the best thing to do. Keep your ears and eyes open; you may learn something useful. Especially, do not criticize. Be strong, and best wishes.