I would look for other options, the first being seeing if the other side of the wall has studs like the post above suggested. I find it really strange that there wouldn't be studs somewhere on one side or the other. Are you looking with a good stud finder? Are you sure there isn't something you can attach to at some point? If there is really no stud at any point, then there probably isn't enough support to the sheetrock to use something like a Ghostud. I've made my own "stud" in the wall just by cutting a larger hole, sliding in a chunk of 2X4, and patching that larger hole. It would be as strong or stronger than the ghostud, but requires a bit of patching.
I also agree on asking the homeowner, but if they refuse it does come down to you.