If I understand the OP's post, he lives in one city, say NYC, and is interviewing with a company in his hometown, say MIA. He was already planning to move to MIA, but the company he is interviewing with in MIA is paying for the round trip ticket from NYC to MIA.

If he is offered the job by this company, he should just tell them he will be staying in MIA and doesn't need to return to NYC as his "roommate" will just ship his things down. He probably already has told the company he is interviewing with that he is originally from there and it may be one of the reasons they selected him to interview. This can speed up his start date as well because he won't need the extra time to move and find a place. That is how I would approach it. If he doesn't get the job, just tell them something has come up with his local family that he can't return on the date they purchased the ticket for and he will find his own way back to NYC. They can probably cancel the return portion of the ticket.