C172 flight instructor reported the student only pressed the left rudder during rudder use training resulting in a momentary loss of aircraft control. The aircraft struck a taxi light before the instructor took over and corrected the inputs.

Narrative

Routine intro flight with a client. We finished our hour of pre-flight and began taxing down taxiway alpha with the intent of taking off from Runway XX. As we were taxing I the instructor was trying to teach to the client how we taxi using the rudders and brakes. I wanted the client to understand how it felt to Control the airplane using the rudders on the ground. I thought I was clear that the brakes are independent and need to be applied together equally to stop straight ahead. As we approach Taxiway 1X I asked the client to begin slowing down using the brakes. Without hesitation the client immediately began to apply hard pressure but only on the left brake. The airplane screeches to the left and the left main tire impacts a taxi light. I was able to correct back onto the taxiway but not fast enough to avoid damaging the taxi light. We immediately begin taxing back to the FBO to inspect for damage. After visual inspection I conclude that the aircraft was undamaged. I believe the event might be the result of a miscommunication and understanding of the independent brakes. But even after the incident while we were taxing back the client still was unable to stop the aircraft straight ahead without slightly veering to the left even after I explained it a second time. I would say to prevent this incident from happening in the future. Instructors on intro flights should taxi even slower than usual and always remain alert to the possibility of impulsive actions.