unless youre prepared and know what youre doing.
but its a major help if you have someone with you. and increases your chances.
The fact that you're asking if you should go alone already exposes some doubt about it. This sounds like a once in a lifetime kinda thing, so I say go for it. Be prepared for anything and everything, and enjoy your time alone, because once your friend gets there you'll be stuck with him.
seriously, kiwis are great people, dont be surprised if you get offered a bed for the night while hitch hiking, coz that happens often with nice tourist backpackers. we had one stay at our house last month. but he was exeptionally nice, and we knew we were saving him 200 bucks to stay at the nearest hotel
as long as youre not planning any extreme walks/climbing expiditions you should be fine.
you will have heaps of fun and meet new people along the way.
oh yeah, we are also one of the safest hitch hiking countries in the world.