Debra makes some good points, you definitely dont wanna be forcing her to eat.
But in my opinion, I wouldn't even worry about counting calories. Just make sure she's eating every 2-3 hours, getting protein and plenty of veggies. I think the worst mistake people make when gaining weight is cut out the vegetables, they help with digestion and well, everything, and certainly gaining weight.
And dont be afraid to "strain" her a little bit, if you baby her she will stay fragile like you say she is. I've seen some women that appear fragile complete some amazing feats of strength.