Yes, I agree that it would be exploitative to actually go to support groups under false pretenses, and that many groups may not want someone sitting in for research purposes. Though, even if you couldn't attend meetings, if you wanted to do hands-on research, it might be possible to contact some local groups and see if the person who runs them would be willing to share general information, such as how meetings are structured and how long they run. Some support groups may have public websites that contain some of this info, too.
I have limited support group experience, myself, but I think a lot is going to depend on how formal/"official" the group is, how big it is, and what sort of leadership is in place. A support group that's a chapter of a larger organization may follow a set agenda to some degree. Smaller support groups might vary more.
My only support group experience is an LGBTQ support group I belonged to for a while. The group was small and pretty informal. There were rules about not sharing what people talked about (in terms of specific things people said. Talking about the topics we covered was okay, I think), and not mentioning other members by name outside of the group. Some people talked more than others, but the group's moderator tried to give everyone a chance to say something, and encouraged discussions.
In any case, what your character is doing would be in breech of etiquette and trust in most support groups, and people would probably be very unhappy to learn that he'd been going to their meetings to use their experiences. If this is part of your intention, then cool. If not, it's something to be aware of.