I know a thing or two about depression, though I won't discuss why. I do empathize with those who have it, or claim to have it (not a subject I'm quick to doubt). That said, pensiveness or tranquility without a goal can feel similiar...that resting point in the climb when it gets too quiet.
I might suggest either a hobby, or working toward a new goal. When I have hit a lull in the past, I elected to work on a new rating...pick something that really has no purpose but to enjoy, and to challenge one's self. Have him go work on a multi sea rating. Or begin building model airlplanes.
Achieving one's career goal isn't really the end of the line, nor the top of the ladder. If he's a new hire, he has a long way to go in learning airplane systems, enhancing his understanding of the company opspecs, etc. He can challenge himself by going deeply into systems knowledge, company proceedures, etc.
Perhaps others have sharper minds, but I've found that throughout my career, if I'm not constantly studying, I'm slipping. Others may be able to rest once they've achieved nirvana, but you'll seldom find me without a stack of 3X5 cards or a study manual in my hands. Perhaps this is something your friend can pursue. Getting the job, like getting a new certificate or rating, is only the beginning. Not the end.