Gaining

If liquids are a problem that rules out my favorite. The ice cream. Milk.Sweetened Condensed milk. Heavy cream. And cake batter shake. It has Never failed for those who have tried it. Massive gains as much as 18 lbs in a week -but those ladies were all 180-200 lbs already with big stomach capacity.
SO. Here are Two other Guaranteed winners. Edible food products that Also Always work.
Pancakes. Plenty of Syrup and Butter. Start with two for a few days. Then three every morning etc etc etc.
The other is for 10:00 every night. Brownies. Make a whole box of them. A tray of Brownies and have her eat all of them in however many nights it takes. In time she'll be able to eat the entire tray in a single night.
Those are Three things that never fail to pack on lbs. Good luck

Why not just eat the food she likes, and let nature take its course? Eating shouldn't be a chore.

Johnxyz is spot on, pick up more of what she already likes to indulge in and pounds will come on pretty easily. Find out what her go to comfort foods are and try to work them into the menu more often.

Also make your environment more friendly to grazing, add candy dishes and stock up on foods she's already prone to mindlessly snacking on.