Are you sure about that bit? I found a 'deactivate all systems' tab. I was thinking I should press that, change my password and then activate the account on my PS3 and have my friend do it. From what you are saying, it seems unlikely.
Just so that you know, the 'my devices' showed 3 systems activated so there's surely a 3rd system activated.
Activating 3 systems at one given time ain't possible. It shows 3 systems, but what that means is that 3 ps3 systems are using your ps3 id and password now. i know it seems confusing, but i am talking from experience.
Leme tell you what happened with me, i got walking dead game from one account (6months back), shared it with one friend from karachi. After he finished it, i though to share it with another friend (omyakha), so i deactivated the systems online. And omy was able to download and play it flawlessly. Now my ps3 with walking dead account activated, got spoiled(couldn't deactivate as it would get on xmb), and i got a new one, tried to download the game and play it, but it showed me already 2 accounts activated, deactivate one to continue. So i am pretty sure what i am talking about, as its from personal experience.
That is what the site is their for, deactivate all accounts, and reactivate on ps3 of your choosing. Give it a try.
- - - Updated - - -
Dude I don't think the PS4 is going to replace the 2nd slot of your account (just like how the Vita doesn't do that either). You'll still be able to use your PS+ account on your PS4 to acquire the IGC even if that account already is activated on two PS3 systems. The slots of PS3 systems and the slots for PS4 systems will most likely be different. So he can still share his PS+ account with his friend, it won't affect his future PS4 purchase.
As it is all speculation at this point, seems like we will have to wait and find out. Though better safe than sorry.