There is no real way to power level someone like in other ARPG's (Diablo).
If you were to join his game as a higher level and start killing monsters, you would be getting a much higher share of the xp for being a higher level while only doubling the base xp gain from monsters. Also if you move too far away from your friend and they aren't able to keep up they won't gain any xp for monsters you killed. Both of these would probably end up slowing down the leveling process for your friend instead of speeding it up. (Wiki page on xp gain in parties)
PoE is definitely a game to be taken slowly and get a hang of all the different mechanics, spells, skill tree etc first hand. Sure you can and possibly have already told them plenty about the game but they still might not understand until actually interacting with it.
I would assure them that yes the first couple acts or so are boring on normal but the game picks up pace once you get more support gems and passive points. Just talk with them over something like Skype or curse voice just to keep them a bit more occupied while leveling and be there to answer questions they might run into.