Ditto. Especially if you have work posted already online and someone recognises the art. There have been cases before where someone wins a contest and it turns out they did not do the winning piece. Even if you say it's ok, the contest holders are going on the basis that the person entering did this piece.
I dunno, have you checked to see if this contest will do a background check to make sure the work is the person's? Either way, I would just keep the art and use it for a portfolio or something.