SSH to only 1 PC

#iptables -A INPUT -p tcp -s -m tcp –dport 1022 -j ACCEPT

*assuming you've already changed the default port that sshd listens to TCP1022

Alternatively add an IP address to the start of your public key line in ~/.ssh/authorized_keys

FROM="Source IP or hostname" ssh-rsa BLAHBLAHBLAH..

Or you could do both.

Setup SSH to only use public key authentication and set a really good password for your Linode account.

He could edit /etc/ssh/sshd_config and use AllowUsers user@ip syntax.