Step 1. Open ports on your router. It's your job to figure out which ports those may be.
Step 2. Redirect said ports to the box in question, hopefully you'll know the ip address that was assigned by your home router's DHCP
Step 3. Make sure rssh is installed.
Step 4. If rssh is not installed, please simply follow the simple instructions to get it installed for your system.
Step 4a. If it's a mac, use brew. Type brew install rssh
Step 4b. If it'a a redhat system, type yum install rssh
Step 4c. If it's an ubuntu system, type apt-get install rssh
Step 4d. If step 4b and step 4c fail, use sudo.
Step 5. Get the external facing ip address, and give it to your friend. Note that this is different than the internal address that you used in step 2.
Step 6. Create an account on your system, using rssh, and allowing sftp
Step 7. Give him the username/password
Step 8. Copy the file into his account
Step 8a. Don't forget to chmod it so he can read it.
Step 9. Tell him to get the file
Step 10. Remove the account from /etc/passwd and /etc/shadow and remove him from /etc/groups. Also delete his home directory
Step 11. Rejoice in the simplicity.