I would either rush to where the accident was, if they called me right away, or meet them at the hospital. Either way, I'd be with them as soon as possible. Regardless of what caused our fight, they're my bestie.
I'd make sure everything they needed taken care of got done so that they could recover with the least amount of stress possible. I'm assuming that, in this scenario, the car accident was pretty severe. If it were just a fender bender, I'd pick them up and help them get a tow, or whatever.
Who wouldn't do these things for their best friend? Even if we didn't talk the whole time because someone was still mad, I'd still help them.