I don't believe much in penance. (The dictionary I checked said "self punishment as a sign of repentance". I don't think either aspect is valuable.) It's not related to the question of how we should treat people when they have conditions that are often under voluntary control.

We should convince them that (assuming they agree that it would be better to not have the condition) their best approach is to accept that the condition is at least partially under voluntary control, that control always appears hard, and therefore to change their lifestyle so as to address the problem. If they agree that the condition is a problem, and they find a magic bullet to solve the problem, then no penance is required. If there's no magic bullet, then they can try to change their lifestyle, but there is no need to for them to punish themselves for not understanding the situation before.