2006 Honda Rancher 400 leaks oil from head bolt

a buddy of mine brought me his bike because its leaking oil from 1 of the two long bolts (10mm head) that help hold the cylinder head down. its leaking right up by the top where the head bolt flange meets the cylinder. haven't had a chance to mess with it yet but he says that bolt isn't catching so my question is, Where that bolt threads into does that seal oil off from coming up or ?? i just figured id ask here first to get your guys 2¢.