Hmmm... this is a good one. On the one hand, if they were traveling separately, it would have been one room for the married couple and one room for the sole traveler ==> married couple pays half. On the other hand, there are many aspects of travel that are on a per-person basis, the biggest being air fare.

I say split the difference; married couple pays 7/12, and the sole traveler pays 5/12.