Moving ipad pictures to a different computer
If you erased the iPad before giving it to your dad, then this issue wouldn't have happened.

Anyways, can you view the iPad like a camera (in windows explorer)? Then can you copy all of the pictures off?

The warning from iTunes about erasing doesn't always mean it will erase the iPad; I've had to sync my device to a new iTunes and It didn't actually erase (but your experience may vary)