There isn’t anything that Apple recommends because the sensors are supposed to be able to be on your skin unimpeded. If she has an allergy to the casing only (as in you have confirmed her allergy is solely to aluminum and not the nickel or other metals on the actual watch back) then you can try a watch cover but there still may be a part of get case on the back that comes in contact with her skin. If doing things like frequently removing the watch to allow the skin to have breaks (don’t sleep in the watch for example), frequently cleaning the watch back and the skin, not using creams or moisturizers on the wrist or waiting until the product has fully absorbed before wearing watch, and any other preventive measures don’t help she may have to stop using the watch at all. Maybe consult with a dermatologist and seek their advice on if she needs to find an alternative to the Apple Watch