When it comes to FC members, trust me, you can have less than a handful of active members and still be happy. I think straight adding members might grant temporary happiness but in my personal opinion, it doesn't solve the root of the problem. Make sure he's okay on a personal level; if he's this brokenhearted in game, imagine how this can impact things irl.

I hope for the best for your friend. I hope he's able to bounce back and be content in his Free Company.