Using SSH on an iMac running Lion
Easiest way in this case is to just


create another user using the Preferences panel
log into this user locally
generate the appropriate SSH key pair
transfer the public key to your friend over a secure channel


In addition you may want to tighten up your system a bit by preventing SSH logins without keys. Open /etc/sshd_config in any text editor and set

PasswordAuthentication no
PermitEmptyPasswords no


Afterwards restart SSH by disabling and enabling remote login in Preferences.