Generally, it works best to have someone other than a family member talk with the person. If there is a trusted friend or spritual leader, these people may be able to help. If the situation is really bad, you can call social services and ask for a welfare check. They have some clout to actually do something about the situation. It's not easy to do, but if the case is serious, sometimes we have to go that route.
Good luck with this,
Carol