There is no evidence that she would want to be euthanized. The word of her husband - seven years after the fact - is not credible, especially given his contradictory statements during the insurance trial, and the clear ulterior motives he has - even assuming he is not responsible for her condition - to do away with her. The fact of his adultery alone is more than sufficient to remove his guardianship over her. If his oath before God and man - his wedding vow - is so clearly broken, what reason is there to believe his testimony on her wishes?
Under the Bible, unless a woman cheated, every man who divorces is guilty of adultery if he remarries, are you willing to go that far? And as far as his motives, do you really think he fears she'll recover? At this point, what other motive could he possibly have other than to carry out what he thinks she wanted? He could slip away into the fog very easily.