I had to give this a google, the lawsuit failed - The tribunal judge stated that “disability discrimination occurs when a disabled person is treated less favourably than a comparator who is not disabled. Those who do not have a disability cannot receive 75% of salary for their entire working life without having to do any work,” therefore “it follows that the non-disabled [employee] is treated less favourably than the disabled, not the other way round.” He received £54,000 a year while on the 12 years of sick leave. Originally absent from work due to mental health and then 4 years later due to advanced leukaemia. My thoughts are that IBM have a very patient and generous disability plan and I worry such use of it may cause them (and the industry) to make it less favourable to people moving forward. I agree with the courts ruling.

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.