Use ps4 friend acc if he has activated primarily on his ps4?

Can I log into my friend's Ps4 account even if he has activated Premär on his ps4?

Of course you can. To be able to play his games it would be better to have primary (ie his with you) and thus you can play the games on every Acc on your Ps4 and he himself the games from his Acc

Because I can't register with his data on Ps4.com or directly on my ps4

Never heard of PS4.com. The thing about registering then he gave you wrong data. You can always register no matter what.

Yes, but when he logs in you will be logged out