Advice for an older woman who needs to gain weight?

I'm trying to lose weight, but my mother is in the opposite boat . . . Her doctor keeps telling her to gain weight. She is 75 years old and about 5'4", but unfortunately I don't know how much she weighs. She's very skinny, though.

I think this is partly a result of her trying to cook low-calorie meals for my dad (who is a bit overweight) and partly because she is a busy bee who constantly "forgets" to eat meals or who thinks of something she needs to do while eating, so she gets up to do whatever and never comes back to finish. She goes to a senior exercise class a few times a week and is very active in general, which is great, but there go more calories.

Are there any foods that are calorie dense and also won't seem inherently "unhealthy" to her? She is very much against foods with saturated fat. And fat in general, really. When I was growing up she would always focus on the fat content on food labels, not the calorie count. I think good fats are exactly what she needs! But if it's in a food that sounds healthy, that would help.