This is easy. There was a verbal agreement at the beginning for 50/50. So the guy should honor his part of it. I think 1/3 would have been fair it that was the agreement at the beginning. Having said that, it does make sense for the couple to share a little more than 50%, may be say 60% as the two ppl would use more utility and other resources. So common sense should prevail and they should opt for somewhere in between...