HELP!! MOTORCYCLE ACCIDENT

on his way home driving his "tricycle", my dad met with an accident. Nakabanggaan nya yung isang lasing na motorcycle driver na dire-diretso lang sa pagtakbo at pasuray-suray pa daw. My dad brought him to the hospital. The man sustained many injuries while my dad only had a few stitches on his right leg. Both motorcycles where damaged and were confiscated by the police. The family of the drunk driver is asking if pwede kami magshare sa medical expenses of the drunk driver. Apparently, they discharged him without him undergoing ct scan. I know for a fact as a medical student that it is standard procedure to have the patient undergo ct scan and if ever the patient doesn't want to he/she have to sign some documents. Yes my dad was partly at fault because he doesn't have a liscence and the motorcycle he's driving is unregistered. Now, we're trying to negotiate with the drunk driver's family. However it seems like they're taking advantage of us asking us to pay for 60-70% of the expenses (medical, motor repair). What legal advise can you give me? should we negotiate or should we take legal action? what are the pros and cons? please help...