Is it true that my account with PS+ will provide PS+ for all other accounts on that PS4, as long as this PS4 is activated as my primary?

Basically, I have PS+ and my girlfriend has no PS+ on her account.

We want to play Black Ops 3 online together in split screen, and she wants to play ESO on her account.

Is it true that she wouldn't have to buy PS+ on her account, since my account has PS+ and this PS4 is activated as my primary?

If that's true, then we've wasted a lot of money :P