Sonic issues the Comtrend Nexuslink 5631 to dual-line bonded Fusion customers. I'm guessing icorey2014
is expecting that some Sonic users may be familiar with this modem/router (not unreasonably so).
OP, the default username/password for administration of the 5631 depends on the installed firmware version. It obviously also depends on whether or not the password was changed during the initial modem setup (which is highly recommended).
If the 5631 still has the original username and password it might be:
1.) no username and password "admin"
2.) username "admin" and password "admin"
3.) username "root" and password "12345"
4.) username "1234" and password "1234"
All of the above are well known username/password combinations used at one time or another by Comtrend and therefore nothing secret. The fact that they are well known however is also the reason it is best to change the password to something only you know.
Please also understand that when you setup port forwarding on your router it may not only let your friend get access but other people too.