Depending on the setting you and your friend choose, if you go to the Wi-Fi room to battle then yes, they do reach Level 100.
And also, in previous games, there were a few Battle Frontier stations (notably the Rental Factory in RSE) that took your Pokemon to Level 100, with Rental Pokemon, however.
When you get out of the battle though, they will not be Level 100. They will still be your previous level.
Hope I helped. :)