My mother suffers terribly from motion sickness but there was no way she was missing out on family boat days while in the VIs. She would agree with all of the advice given, especially the nevar37's mention of proper dosing levels with meds.

Non-boaty friends did full days with Dive BVI and they were fine. But again, they wanted to come along and didn't feel obiligated to attend.

What is the famliy dymanic? Is she expected (via your son) to come along and make the best of it? Or your family ok with letting someone do their own thing once in a while?

I would start with a conversation directly with the girlfriend. Tell her the group plans and let her decide if she wants to participate or not. If she does, send her here for questions and adivce!