Funny, I was just talking to someone on Etsy about this exact problem because they were looking for a product from my shop to help their son.
Chronically dry skin that doesn't respond to lotion is a sign of a systemic problem- no cream is going to help. Lots of time is is a gut imbalance, and something as simple as cutting sugar and refined carbs and/or adding daily probiotics can help. But it can also mean deficiencies in certain micronutrients, specifically the B vitamins, the fat soluble vitamins (A, C, K, and to a lesser extent E, but it's hard to be deficient in E because we don't need much), zinc, iodine, amino acids like glutamine, or a number of others. I would suggest exploring those avenues next. Adding a fish oil might be a good start. Also coconut oil (eating it as well as using it on the skin- my kids eat a spoonful a day in the summer). Nutritional (aka brewer's) yeast is a good source of B vitamins, and kelp is a good source of both iodine and magnesium. Those would be a few things she could start with. Hope she figures it out soon!