Your dads balls are worn dude....
No seriously, he needs a valve body replacement on the gearbox, it runs on plastic balls that over time thin out, hence, worn balls.. do NOT, I repeat, DO NOT drive this car, if you engage more then one clutchpack at a time, the box could go poof. Thats the differance between a 5k repair and a 25k repair.
I can remommend Dennis from Gearbox Den.