Howto: Forward your SSH port
They will need to create you an account on their box
This assumes that you're running SSH on port 22.
ssh user@their.box.com -R3333:127.0.0.1:22This will forward your port 22 to them, so that they can access it at 127.0.0.1:3333 on their.box.com.
Then you just need to give them access - assuming you trust them.
If they're fixing a problem of yours, then I'm assuming they need root access. Either temporarily change the root password, or add their SSH key to /root/.ssh/authorized_keys2.
Of course, if you're both behind NAT, you're in trouble. Get IPv6 working, and let's get rid of NAT forever.
This works for other TCP services - not just SSH. You can forward http, ftp, telnet, whatever you like. Just change the port 22 for the correct port number.