My first reaction is 'Clutch', even though you say it 'seems' OK.
Clutches wear out a lot more readily than gear-boxes, (in fact they are DESIGNED to wear, three plates held in contact by friction, one of the plates is made of soft 'friction' material, between two solid metal ones, so THAT one wears rather than the metal & is more easily replaced!) and when boxes wear out they go 'sloppy' tending to drop into gear a lot more easily or, if very worn, falling out again, even MORE easily. Stiff selection, could be lack of lube, but much more likely the clutch.
People have a habbit of 'riding' the clutch, particularly at traffic lights and stuff, rather than using the hand-brake, and many dont even realise they are doing it!
And driving the car every day, they never notice how the 'bite-point' of the clutch is getting closer and closer to full pedal travel.
Anyway, first point of call is to check the clutch fluid, make sure that the clutch is actually getting full travel from the pedal.
Then the gear-box oil, to be sure it is lubed.
But more than that needs some mechanical archeology, and if your dad isn't a confident DIY mechanic, will need booking into a garage for diagnostics, or some-one off here whose more mechanically minded to have a 'poke'.