What to do about dog that is underweight?

I have a dog that I have been doing drop in visits for the past few months. The dog was adopted from a shelter at least a year ago and is underweight (can see all ribs, hips protruding). When I first met the dog I figured the owners were helping out weight on him. He looks the same though and I am concerned. The owners love him and don't intentionally seem to be neglecting his care. It seems very possible that this is how they got him from the shelter and think he should look, since he is full grown. They seem very "new" and this dog has no rules, the dog runs the house. He gets fed once a day at night (so he doesn't wake them for breakfast) and they always talk about how much exercise he needs.

How do I go about bringing up that their dog needs more food/to gain weight? I am embarrassed to walk him because he looks malnourished. And obviously he is not healthy so that's my main concern here.

I told a friend her dog was too skinny in the past (he was, visible bones, and she got very defensive and upset, so I just want to make sure I go about this politely).