This is a wonderful question! No one ever thinks all that much about Mr. Mallard. I am going to speculate, based on the story itself and the time in which the story took place, that Mr. Mallard, had he known of Mrs. Mallard's thoughts, would have been at least mildly offended and assumed something was wrong with his wife. I find it highly unlikely that he would have been sympathetic.
The part of the text that makes me think this is as follows:
There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature. A kind intention or a cruel intention made the act seem no less a crime as she looked upon it in that brief moment of illumination (para. 12).
This passage tells us that Mr. Mallard had a "powerful will" that bent Mrs. Mallard, not allowing her to be herself. We have no reason to think he was a deliberately cruel man, and we know that she loved him sometimes. But Mrs. Mallard thinks of this as criminal, the complete repression of another human being. And her response to freedom is so joyous and overwhelming that it suggests an almost emotionally abusive spouse. I do not think that such a person would have the empathy to understand Mrs. Mallard's feelings or the insight to examine his own behavior. He would not have been pleased, and it is possible that he would have assumed she was mentally ill, as the husband does in "The Yellow Wallpaper" (Gilman), a story taking place in the same era, published in 1892.
This story was published in 1894. I am not sure that young women today realize how few rights women had in that era or how much they were dominated by their husbands. Wives were almost like children, to be cared for by their husbands financially, and like children, all major decisions were made for them. I suggest that Mr. Mallard's behavior, even if it did approach emotional abuse, was the norm for the era, not the exception. Thus, he would not have been pleased of being accused of domination, since this was typical husbandly behavior. And there were women in that era who were actually involuntarily institutionalized for not behaving like "proper wives," i.e., compliant and grateful.
Today Mr. Mallard would be encouraged to seek marriage counseling or Mrs. Mallard would simply seek a divorce, but the story, reflecting the reality of the times, would never have included such solutions.