Here's what happened to my Hosta montana macrophylla. The sunburn appears to have caused a weakened leaf structure which was then pummelled by heavy rain last night making a bit of grotesque sight in the garden.
There is really nothing that can be done other than remove the most damaged leaves - ultimately this should not harm the overall health of the plant. Even though I've had some heat damage in years past on these big leaves - it has never been this bad. Perhaps it was the fact that it had been so cold and the change in temperature so dramatic - 20C in just one day - the leaves simply hadn't had a chance to toughen up to ready themselves for the heat and sun.