2002 Ford F 150 Dads Clutch Leaking Fluid Parts Repairs Made Cost Such Puts

my dads clutch is leaking the fluid so what parts and repairs need to be made and cost of such if he puts more fluid in it runs right and this is recent that he had no problems prior and just one day had problem with clutch when checking fluid there was none added fluid and within a short distance resivore is empty again does he need to replace the clutch plat and other parts or can it just have a leak. i am worried that someone take advantage of him being a senior citizen he was told by 1 mechanic without looking at vehicle that it would cost 500-600 dollars