I asked on his behalf because nobody could tell him how he overstayed his status and what to do to turn it into a visitor. Everyone keep mentioning a new visitor visa, but that doesn't apply to him.
He should have applied for a visitor record before his study permit expired. This is the same process anyone else would follow in Canada no matter what country they are from. US citizens follow the same process as anyone else.
When he didn't do the above and fell out of status, he should have applied to restore his status within 90 days to a visitor record. Once again, this is the same process anyone from any other country would have followed. US citizens follow the same process as anyone else.
This was honestly an extremely straight fowrard situation and there's really no excuse for falling out of status. If he was confused about what to do, then should have hired a lawyer to help him.
Is there a reason why he didn't apply for a PGWP after he completed his studies?
Of course if he's in a common law relationship with a Canadian citizen and that Canadian is willing to sponsor him for PR, then another option is to remain in Canada without status and submit the sponsorship application. Of course he can't be working right now since he's in Canada illegally (hopefully he isn't working) and it will be quite some time before he's able to work in Canada through spousal sponsorship. This is an option if he's willing to remain in Canada a long time without working and his partner is willing to financially support him through this period.