I think they would definitely be responsible and I would not feel bad about suing them. People who sue if someone barely taps them is one thing, but he is actually hurt. They are responsible and medical bills are ridiculous, the responsible party should pay. Was a police report filed? I would think there might be problems if there were no witnesses, I would assume you must prove your BF was obeying the traffic laws.

Best of luck with getting the responsible party to pay and a speedy recovery for Jim!