ssh into lan behind router
One friend of mine has asked me to check something on two linux machines he has on it's lan which is behind a router/firewall. As details it only gave me the two private ip's of them two linux machines to which I should connect by ssh, the login details(username/password) and the external/public IP address. I believe that it should work on standard port 22, as he didn't mentioned any other port settings. NMAP seeis that his router has port 22 opened(filtered, actually).
I can't talk to him now as he's not available.
Any idea how should I get into each of those linux servers by ssh?