How does the mother react on the last wish ?
The mother's reaction to the final wish is clearly one of devastation, as we see within the short story's final sentence: "a long loud wail of disappointment and misery from his wife." Her feelings are a reaction to losing her son Herbert again, after having believed that the monkey's paw offered a chance to bring her child back from the grave. Those feelings are magnified by her having felt so close to seeing her son again and then having those hopes dashed so suddenly, as well as a realization that the loss of her son is indeed permanent.
It seems clear given the timing that Mr. White's last wish must have been to undo the second wish, thus sending Herbert back to the grave. The fact that the incessant knocking ended as the door was thrown open, yet no one was there, gives credence to this idea.
Finally, Mr. White and his son Herbert are playing chess as the short story opens.