Good Morning - at least here in Tumbleweed country,
Your Dad's lenses, as everyone else has indicated, will fit on Pentax's digital bodies. Pentax still uses the K mount. Also, just in case he has any old M42 lenses (the screw mounted lenses from the Spotmatic days), they will also fit with an inexpensive adapter from Pentax for about $30.
Obviously, given the age, they will all be manual focus, and like Marc said, there is a great set of instructions at the top of this Beginners forum on how to use them.
Being able to use the old lenses is one of Pentax's trademarks, so there are a lot of folks that hunt for old glass to use.
Also, there is a wonderful site that has information on all of Pentax's lenses - past and present.
Pentax K-Mount Lenses and Lens Accessories
I think that your dad may have a lot of fun shooting digital - since it is pretty immediate as opposed to getting the film developed.
hope that helps....