Is there any way you could amputate her feet or surgically remove a few vertebrae? Liposuction? That might help free up some space in the bed.
This reminds me of a question i saw on reddit where some guy said his dick was too big for his wife's mouth, and did anyone have any suggestions. I asked if he was already circumcised, and if so, perhaps his wife could have some of her teeth removed.
In either case, it would probably depend on how good your insurance is, especially in your case, since buying a king size bed would probably be cheaper than elective surgery.
Other than that, what about just using the floor?
Also, i don't know what type of mattress you have, but the most expensive futon is cheaper than the cheapest spring mattress, in case you weren't aware.