You didn't mention if this was a long term problem, or one that started recently.
Why are they staying with your family for a couple of weeks? Is their family under some sort of stress and without a home right now? That alone could cause a child and their parents to establish unhealthy patterns with food and sweets. The parents feel stressed and guilty about something, and are worried he won't eat OR the child feels stressed and doesn't want to eat, so again, the parents are worried he won't eat. As a result they either force feed (which never works and only causes a power struggle/more problems) or they give in to allow unhealthy choices (too many sweets/fatty foods/same food over and over, no veggies, you name it). This is the parents problem to work on. They have to establish healthy eating expectations and be consistent.
If it is a short term stressor, they need to recognize the stress and start talking about it. He can only use food as a way to feel in control for so long, before this stress will trickle over into other behavioral problems.
If it's a long term problem, and they've just established bad habits by being inconsistent, you've received plenty of good advice below on how they can start working on the problem. e.g. end the force feeding/power struggle, start eating together as a family, avoid endless snacking, reward with activities/time spent together not sweets, etc.
Regardless if this is situational or a long-term problem, he's old enough to have a proper conversation about what's OK and what's no longer allowed. They need to explain that changes are necessary so he can give his body and brain everything it needs for proper development, and to keep him healthy and strong.
Good luck!