Its a tough one, and people can often react badly especially when there is blood involved. At the very least calling the cops or an ambulance (or both) would have been a minimum!
I have been a few situations where I have pulled over and provided assistance, and often do it with very little hesitation, but since I have had children its not as easy to jump in boots and all.
I once came across a lady having a seizure on the side of the road, just laying on the grass verge fitting. I pulled up, left O in the car with the windows open (right next to me) and called an ambulance straight away, then looked after her until it came. When she stopped fitting she had no idea where she was or anything like that, so am really glad I stopped to help.
I am a very firm believer in karma, so for me it would be really hard not to help someone in trouble.
I hope your brother isnt too shaken up and that his friend is okay.