I agree with the others about the differences with the mentioned lenses, but my 40mm is sharper than my nifty fifty. Maybe it's my particular copies, not sure, if it even matters. Honestly, 10mm is slightly noticeable when comparing 40mm to 50mm. If a person wants to use either lens for portrait work, the 40mm will be a bit tighter.
I like my 40mm more than my nifty fifty. I think the nifty is not very sharp below f/2.2, but it is still usable. The 40mm's auto focus is faster and more accurate than my nifty fifties' auto focus. For what it is worth!
OP, maybe you should ask your dad what he wants to photograph and go from there. You don't have to tell him you are buying a lens, casually ask him what he plans to photograph.