"Caveat emptor" ie buyer beware.
Unless you misled him, he has no claim.
"A mate of his checked it" - a court would probably query the skill / qualifications (any instrument is only as good as the operator using it, plus "measurement uncertainty", ie temperature / humidity, calibration and other factors play a part in it too). A court would also pick up on the fact it's 8 year old, thus entitled to fair wear and tear.
Indeed, if it were me, I'd let him take it to court, and argue the point that an inspection should've have happened BEFORE, and not after, agreeing a sale.
A point that keeps coming up on bike forums (esp with newbies buying their first bike) is "if you're not sure what you're looking for, take a clued-up mate along" - to prevent this type of situation occuring - which is what your buyer should've done.