I have a bad back and these beds are not bad. I find them more comfy than some hotel regular beds. They are not big, so if your relative is sharing with a little child, they should do fine. When we rent a studio, I sometimes sleep on the sofabed with my little daughter when she and brother ar at each other's throats. I am usually so tired I conk right out and wake up OK the next morning. You guys should do fine, new mattresses or not.
PS: Just so you know, virutally every building at OKW has handicapped parking. These spots will be close no matter where you are assigned. You might mention at check-in (that a member of your group has a HC placard) because some units in the individual buildings are closer to the handicapped spots.
Have fun!