He can play on both PS4s with the same account.
If he owns a physical copy of the game, he just needs to bring his game disk with him.
If he owns the game digitally (purchased on the Playstation Store), he'll need to be connected to the SEN (Sony Entertainment Network, formerly known as PSN) to play. He can only play offline on his Primary PS4.
Note that even if the PS4 is connected to the internet, if the SEN is currently offline, he won't be able to play his digital games on the secondary PS4. This has happened multiple times in the past, particularly around Christmas and New Year's.
If he intends to stay over between Christmas and New Year's, he might want to deactivate his Primary PS4, and activate the other one. He can always switch back afterwards without losing any data.
If he forgets to do so, he can also deactivate his current Primary PS4 over the internet, but only once every 6 months.