Yes, in order to play online, you both need PS +. But there's a trick how you can both have PS + with just one account. This feature is called "Primary PS4" or "Share Play". To share your friend PS + without any disadvantages, you have to do the following:
You give him your account data (from the account with PS +)
Now he logs in with his account on his PS4 and goes into the settings
Once he has done that, he has to search for "Primary PS4" in the settings (This item can be under another, i.e.: In the audio settings there's the sub-item Microphone)
If he finds it, your friend will go to Activate Primary PS4
Now you have shared your PS + with him for free. If he is now in another account or in his, he has PS +, although only on the other PS +.
Important:
Be careful who you give your account details to. It can often happen that this is taken away from you. One more note: if he has activated the primary PS4, he can also download and play games that you bought in the Ps Shop as long as he has activated the primary PS4. If the primary PS4 is gone, there will be a lock before the game and it can no longer play. (The game you're playing)
Also important: if you give him primary PS4, you can't take it away from him until he decides to turn it off. Another option would be to change the password. So he loses access to the primary PS4 because he has to register again.
If you don't quite get it, search for "Primary PS4" or "PS + share" on YouTube
