I don't think mileage will be an issue. Full synthetic oil should be able to go 10,000 miles. But if the car isn't being driven, then calendar years is something to consider. 1 year sounds reasonable. It's just oil which he wants to use and not waste. I've done that myself. Poured a few leftover quarts of oil I had into my car; mixed in with the new oil. 2 different weights.
No harm done. It's all fresh out of the sealed bottle.
No harm done. It's all fresh out of the sealed bottle.