As we all know Simon had been transfromed by the crown into Ice King
What if Betty was transformed herself but the magic done to her kept her in a form that would be a curse just like her love.
While he has a body of a humanoid with powers starting to form, Betty flung away from him far from the blast zone turned into a small creature unable to speak but a strong will to be with the man she loved maybe even herself unknow to who she is..
He would probably die regardless, considering the amount of radiation he undoubtedly lived through being exposed to only because of the crown, not to mention his body temperature and living conditions. 8'I