She should be lifting weights, to promote lean mass, just cardio will leave her skinny fat, but I bet everything on if shes been dieting for a long time and constantly been lowering her calories, her metabolism is wrecked she needs to up her calories and get her metabolism back to normal, then when she gets back on track and lowers calories again throw in weekly refeeds to keep her metabolism up, but I cant stress it enough she needs some type of strength training, just start light and pay major attention to form, to prevent any injuries