You can not claim a deduction for personal rent on your tax return.
I assume that the friend has signed a contract as the lessee and that you are not named as a co-lessee.
Your friend is a sub-leaser and might actually be required to file a schedule E on his tax return however I would take the position that since his income/cost and/or expense would all be $5200 then I'd probably not do anything with it depending upon your relationship and how much he is paying for rent and utilities.
His income is known but his expenses are highly debatable.
Floor space as measured by square feet. What percentage is common area and private area to each?
How much time is each person there?
Does he supply furniture used by you? TV? kitchen stuff or appliances? Are you supplying these things?
it could go a long ways beyond these questions.
Under audit I am sure that the IRS would not allow him to suffer a financial loss because to do so would allow him to write off his own personal living expenses. He could probably show income but not be allowed a loss.
By the way, if you have had a falling out with the friend the IRS is not an instrument of petty revenge. They take steps to insure that they are not used as a petty revenge instrument.
One more thing; it is the friends problem not yours. This has probably gone to tax court in some case that I am unaware of. I've never had this question come up with one of my clients in doing returns for over 20 years; your friend has an interesting scenario. I'll check back and see what others think; don't take this answer as the final answer. Also he should get his own tax preparer and ask about this in a place where there can be two way conversation to get the facts out.
You have a really good question.