Hello,
how is the things in family. I hope better.
No, Depression is a very complicated matter. At my opinion it is almost uncurable. It isn't related to Sugar uptake. The roots is that the person feels inadequate and desperate.
So don't ask them anything like restrick Sugar intake. It will make things worse. Actually don't demand anything from them.
The only thing that can treat Depression is the person's will to cure him/herself. The key is to find what is that which make him feel desperate and to resolve it.
Also he has to start having dominant activities, because Depression makes one very submitive that's why they see TV all day long. There is one book which is very helpful. Talks about the human brain mechanisms besides depression. I ll try to send u a link.