If the interior is flawless, dash has no cracks, engine compression checks out, no mystery leaks, it might be worth that much. I'm not sure anyone will hit him with an offer though. If you don't know these cars, 319K miles will scare anyone off.
See if he can document the service history. I'd hit him with an offer of $2k cash...or some sort of cash & trade offer. If he refuses don't be afraid to walk away, I've seen a lot of great M42 E30s go for about $1500-$2000.