It's likely nothing more than the external linkage between the lever and the transfer case is binding or out of adjustment.
Crawling underneath to watch the t-case lever linkage while he attempts to shift it will probably tell you enough to figure out what the problem is. Odds are it's nothing more than the linkage (perhaps it slipped out of adjustment) and nothing wrong with the t-case itself. If it's out of adjustment, there's just one bolt to loosen and one shift rod to slide back or forth until its back in adjustment, followed by a healthy tightening of the bolt to hold it in adjustment.
It's easy to see how its done once you're underneath to watch how it works.
