I think soups, porridge, mashed potato - any little snacks seem to go down well. Finely chopped or grated salad items in sandwiches - with maybe some egg or cheese too are much better than a big plate of anything. Think half portions - and more meals, perhaps three-hourly.
A snack with a cup of tea counts. How about making fridge granola bars (recipes online) and cut them into little one-inch squares. They freeze well. I like them made with almond butter, oats, Sweet Freedom cinnamon syrup, chocolate chips and chopped raisins.
My friend's mum is never hungry, so is easily put off by a plate of food. She does eat, however, if she's at the table in a 'Help yourself to anything' social situation. It doesn't help that her teeth are loose and she won't see a dentist.
Unfortunately, she dislikes milky or thickened drinks, but does like hot chocolate. Last time, she ate mini quiches, finely chopped beetroot in mayonnaise and potato salad - with (strangely) vegetable crisps. It's all nutrition and calories, after all.