My father has rented a move on iTunes, but on his old computer (which can't cope and crashes when he tries to play it). He's now on a newer computer, and logged into iTunes with the same account, but we can't find the rented movie.
I can confirm that the following works:
- I downloaded a movie on my pc-laptop
- I moved it via my network share, added it to the library on my macbook
- It was visible as rented there but could not be watched (authorized to another computer)
- Then I unauthorized the computer I downloaded it on and it worked on my macbook
You should move your dad's entire iTunes library from his old computer and merge it with the iTunes library on his new computer.
Read Apple's technical support documentation article: iTunes: How to move your music to a new computer