Gaining

make one she will like. Whats her favorite ice cream flavour? grab that add milk and heavy cream and have that daily and she will start to gain over this month.

Just add more ice cream or chcoclate topping until she likes what it taste likes.

You could also persuade her to go on a strict diet for a few months, that will send her metabolism in to starvation mode and when she starts eating normally again she will gain all her weight back rapidly and keep on gaining easily. Not soo healthy but thats what diets do. Big business the world over all this yo yo'ing