At her age this is a marathon not a sprint, to that end I would go with delta, she will begin the slow journey to million milier, if she moves there is a greater chance of delta having a major presence than American, most of the credit cards that come with lounge access are probably out of budget at this point in life but they won’t be in a few years in the meantime she can get a cheaper card which at least helps with mile accumulation, if she was at a point in life where she is charging a few 100,000 a year on a credit card aa might make more sense as status is easier to get via credit card spend, but my best advice is to think long term as she begins her journey