I don't have any second skin, but I would cover about 25% (more than 25% and it stops working as a vibration dampener and becomes the same as adding mass which it performed better by LLP) of your sheet metal in the car with damp pro and then a layer of luxury liner pro through out the entire ride. The damp pro prevents the metal from vibrating and the llp blocks out external noises like road noise.
As far as her ride, I would find a nice 8-10" and put it in a sealed box either glassed under a seat, or into the side panel, upgrade the stock speakers to a nice set of components and be done with it unless she wants a better HU which isn't a bad idea.
For his ride, I would step it up to a 12", again sealed, with some higher end components, amp for mids/highs and something like this so that his stock HU can bu utilized.