Slipping on ice, while on the job.

Indiana. My father was delivering his product to a major retailer. While stepping out of his truck, he slipped on ice, causing him to fall head first on the parking lot. He spent 2-3 days in the hospital with a blood clot on the brain, and almost a year of rehabilation. He has problems sleeping now. His whole attitude towards his job has changed. He know worries himself sick about the weather. If he goes near the store where he fell, it makes him sick, breaks out in cold sweats while in the store. After the year of rehab, W/C has said that they think he is OK to return to work. Even though that he cant sleep at regular intervals, and he is on the road at 2 a.m. W/C has offered to settle up for around 5K. The two questions that I have are. Does he have to accept W/C offer? Is this amount set in stone? The other is, does he have any recourse against the retailer for his fall on the ice? If so, and he filed a law suit against them, could his employer fire him for that? Thanks for you time.