Oh I agree that he should pack at least a pair of long pants, even if it's just jeans.
That being said, since the return from the Covid shutdown, there are more & more ships that now accept shorts as acceptable attire on casual evenings, even though it's not indicated in their write-ups.
If the guy finds out that shorts are good to go, then he'll be a happy camper, if not he can always to to the buffet.