UA: Good because is pretty fast getting status on it, if your friend is planning on flying more than 50k miles a year, I would get UA. UA credits 100% STATUS miles for a lot of cheap Y tickets where you would get between 0 and 50% miles on other programs.
After reaching 50k status miles, he would get Star Gold which is great.
A down side is that spending your award miles on other *A carriers is pretty expensive (mile wise) but it's good if he can spend them on UA tickets.
NZ: Don't know much about their program but status is hard to reach I think. Can't say much here.
LH: Wouldn't go with them unless he plans to fly expensive Y tickets. Earnings are ok on LH, SQ but unless he will fly from NZ to Europe every month, reaching status there would be very hard (Senator status at least, Frequent Traveler status would be useless being in NZ). Wouldn't go with them.
Other options:
OZ: He can reach star gold with 40k miles within 2 YEARS. Status pretty easy to reach. Don't know much about redemption, etc...
TK: Status a little bit harder to reach than OZ, it is 40k or 45k within a year can't remember but is very easy to keep your status for years once you get it! Spending your miles on award tickets is very cheap as well (one of the cheapest out there I think).
Overall, I would go with TK or UA. I would avoid LH/NZ if he is planning on booking cheap Y tickets. And I would consider OZ as well.
But ultimately it will depend on which carrier he plans on flying and the booking class.
From NZ to Europe he can fly:
NZ: cheap Y tickets earn 0% on almost all programs
SQ: you can get 100% mile earning tickets on TK's program for relatively cheap so that's great. If he chooses to fly with SQ on 100% earning fares on TK, he could get Star Gold status with 2 return trips!
TG: Don't know much about them
LH: LH doesn't fly to NZ so he would need to hop on SQ or TG first then LH. Earnings of cheap LH tickets are pretty bad, would not go that route.
If it was me, I would enroll in TK (or UA if he plans to fly to the US at all) and fly SQ to get Star Gold with 2 return trips. Plus SQ is the best * carrier to fly between NZ and Europe ^