Tell your mom to eat more, if she's used to four eggs, tell her to eat 6, an extra helping of this, and extra helping of that... mo' butter too, mo' butter.
Basically, gaining weight is about eating more, creating a calorie surplus, not sure how hard her training is, but she needs to not only make up for those calories lost during training, but eat slightly above what she may have lost. If she's eating primal she should put on some weight without packing on too much body fat. Is she low-carbing? I wouldn't have her worry about that, as it can get in the way for gaining weight.
I wouldn't advice eating too many beans and lentils, but there's worst out there.
I forgot to mention, give her a hug for me and tell her it's downright fucking awesome that she's doing this at her age.