15 years is a long time to keep doing something he hates. Surely the pension rights he has accrued will stay in the company scheme even if he moves job? So he'd get 3/5 of that full pension, or whatever the proportion, plus a proportion of whatever pension scheme his new job provides (or a private one).
He may be in a final salary scheme which is now closed, so any pension he'd accrue elsewhere would n't compensate and presumably it is a long time since he started a comepletely new job, so a daunting propsect. Yes it seems a shame but security is more important to some.
I know someone who persevered with the job from hell for 20 yrs because of the pension. He pretty much hated every day he worked there. He was made redundant 6 months ago yet still has 25 yrs left of work. He is at a loss what to do with himself, can't find a job and doesn't even know what he wants to do.