You want the xp items for fast leveling, then just mp10 keep over and over. Generally people ask you have hellfire ring and cain's set and also possible leoric ring, as well as a high ruby. Can't really buy a hellfire unless you're 60 though, can buy a cain set for like 100k total so your friend could probably buy you a shitty one, just make sure to get a socket in head piece(if you buy the headpiece) for the ruby, and if he had a high lvl ruby he can lend you. Leoric's signet is expensive as shit I believe.
Don't know how slow it is but having just leveled my monk to 60 by myself it can't really be that long, I was getting 1 level per keep 2 clear at 55 on mp0, that's like 5mins or so. Had cain's set, hellfire and a shitty ruby cause I'm poor, was using tempest rush with a 1000DPS 250dex mace though, running around at like +70% speed or whatever.
For a DH powerleveling I think he'd want lacuni+movespeed boots+strafe with the 100% rune, and he should be able to kill everything around him instantly while moving so it'd clear a lot faster than most other setups.
