Hospital parking fees can be brutal - even worse when you need to go several times for several hours. Could you buy FIL a several day parking pass for the hospital your MIL is at? Then at least he won't have to worry about how long he stays with MIL at the hospital or how many times he can visit & hopefully won't have to park miles away.
This is a great idea if it's available. My dad has just gone through cancer treatment & subsequent surgery and whilst he was lucky to have all his parking costs covered (he's a Vietnam veteran) it was costing me a bomb to visit him each day. So yes, visitors parking would be a huge help for them😊