If you don’t have the DLC and especially on PC, the mother of Grogans in the Anvil is a pretty quick farm. Thing is, she dies instantly from any melee attack so if you have a facepuncher you can just go and one-shot her over and over again. (Disadvantage to this method is that you have to savequit inbetween kills which, particularily on consoles can significantly stretch the farm out. On the plus side, you don’t need to worry about the farm getting slower as you eventually outpace your gear because you are getting a guaranteed one-hit every time)
There’s also a sidequest on Athenas you can do which awards an insane amount of xp upon completion (about 1 levels worth).
As for your friend, I would still recommend letting him do the campaign in normal mode, because that unlocks Mayhem Modes, guardian ranks, the Takedown etc. Once he’s done with the story he should be around lv. 40 or so, at which point you can PL him to 53 (or rather, 57 considering he probably won’t finish it until tomorrow).