Fattening others

Try to get her to eat a little more at each meal.
If she likes ice cream try a bowl of that right before bed.
She doesn't have to stuff herself or anything like that because gaining weight takes time.
The more she eats the body will adjust to it.
Take it slow.

It helped me to gain weight when I couldn't eat very much at one time.

Calorically dense foods are a big help. Especially
high fat ice cream.