Plastic deteriorates over time especially if it has been exposed heat and cold. After a certain amount of time, it is impossible to know how it would perform in a crash. The stress of a crash could cause the seat to crack and break.
In an accident, even if there is no visible damage the the force of the accident could cause micro fractures that could cause the seat to break in another accident. My husband and I have been in 2 accidents this year that totaled both of our cars. Both times the kids were in the car - no one was hurt. Both times car insurance covered replacement car seats - once the other driver's insurance and once our insurance. Insurance would not cover replacements if there wasn't a lot of proof that car seats are only safe for one accident.
If your dad doesn't want to spend a lot on a car seat, he can get a cosco scenera. It's very inexpensive and has a very good safety record. It would be a lot better than a used one where you don't know the history especially if was only used occasionally.