Was on a road, and he did check his mirror and he wasnt there, the van come out of a turning, the van was speeding and did have more than a doors width of room
Usually if you are encroaching into the flow of traffic its your fault. For example if you pulled out in front of somebody from a side road. The vehicle alreasdy on that road has priority. You could argue a lack of due care and attention from the van driver IF the road was clear of vehicles when your dad opened the door and the van has exited a junction onto the road your dad was parked. Therefore the door was already open when the van approached it so would have seen it had he been paying adequate attention.