There’s all sorts you can try (most covered already) and i’ve had the misfortune of trying most of them. Worth asking the doctor if they can get you/mother an appointment with a dietitian as they’ll have loads of tips.
I found fortijuice and ensure plus impossible to drink in enough quantity. You can however get prescription for a carbohydrate powder that can be mixed into almost anything and bottles of liquid fat that can be mixed into stuff like mashed potatoes.
My personal favourite tip is ‘super’ milk. Take full fat milk and stir in instant milk powder, tastes much the same and boosts calories of anything with milk in.
Depending on how bad things are, and any underlying medical conditions, some sort of tube feed may be far from the worst thing in the world, and a few weeks of good nutrition can really pick you up when you are run down. GP/Dietitian are of course best qualified to advise on this.