Without looking into it, just going by the information you gave...it seems ludicrous on the face of it, like selfish, entitled, etc., but it could make sense. If the man really was sick all that time, his sick leave is probably the only source of income he has or can possibly get, to keep him off the streets (unfair as it may be for the company, but then who cares--companies are evil, exploitative profiteering machines), and 15 years makes for a lot of inflation. It's probable that what he was paid 15 years ago is no longer enough to cover the cost of living.
My other thoughts: he's lucky he was able to pull off getting sick leave for 15 years, and by suing them, he may be pushing his luck...they may find a reason to fire him.