I would recommend following up with the buyer. Let them know you are happy to hear they got the product to work after providing the instructions based on the product review they left. I would them ask if they would reconsider removing their negative feedback.
Amazon has really tightened up their removal policy to only do so if it qualifies based on the standard reasons for removal. Your best solution would be to get the buyer to remove it. They may have thought that leaving a positive product review would take care of the feedback.
If, after a few days, they do not respond or remove the feedback, I would recommend you respond to the feedback so others viewing it will see your response.