I can't join this server?

I'm trying to join my friends minecraft server, it either says it's full (because it thinks I'm still in the server) Spawns me inside a rock underground, or says I'm dead and doesn't let me respawn and I don't have my 3D part of my skin on my head Incase thats important, and everthing is frozen exept me, I can just walk around and nothing moves and nothings happens.(When I mine out of rock)

His Server is slow and its causing entity lag. 90% sure its the server's fault