I'd bet (no, I'm not a medical professional) the medical condition that would cause such a thing would be considered a marvel. Even conditions like Cushings syndrome or hypothyroidism, where "extreme weight gain" can be expected, would consider an extra 100lbs to be exorbitant and cause for immediate medical treatment.
And if that person happens to have a such a problem, I'm sure it could be worked out that they dont' have to pay extra. My guess is they won't run into that situation often.