Actually you can: Heavensward will allow players to enter ARR dungeons(and pretty sure trials too) without (i)lvl sync, as well as with less than required players. So you could go in with your friend and wreck face, and even get gear! The only problem would be no exp from mobs (since even if they don't disable that a part diff of 10 or higher levels stops such), and possibly exp from completing the dungeon is also disabled, but there will still be plenty of fates and the like to do.

Also you technically have to use the duty finder to get into dungeons even if you are in a pre-formed group, so no worries there.