I've never had to gain weight, but from what I read: Eating primal and focusing on strength training is the way to go. Obviously she'll want to eat over her required calorie requirements, or make sure to not workout so much that she's in a deficit. She'll want to be at a calorie surplus and through eating good wholesome food she should see gains without adding too much body fat.
I'm no authority, so feel free to correct where wrong.
I used to seriously post here, now I prefer to troll.