Flight Instructor reported a brief runway excursion after the passenger inadvertently engaged the brakes during a discovery flight. No aircraft damage or injuries resulted.

Narrative

I am a CFI working at ZZZ. I took a person on a discovery flight they had purchased. The plane that was used was owned by [the FBO] and was an Evektor Sportstar. Before and during the flight; I briefed the person on various safety procedures including the three-way exchange of controls and keeping his feet on the ground during take-off and landing so as to not touch the brakes. During the final landing at the end of the flight; I had control of the aircraft and landed on Runway XXL at ZZZ. Immediately after touchdown; the discovery flight customer held the left brake; sending the plane aggressively to the east side of the runway and into the grass. After several attempts to tell the passenger to take their foot off the brake; they released the brake. At that point we were about 10 ft. into the grass; I choose to keep the stick back and slowly brake so as not to slide on the wet grass. We safely made it to the taxiway turn-off after paralleling the runway in the grass for what I would say is 6 seconds. There was no visible damage to the plane and both the discovery flight customer and I were completely fine with no injuries. I believe what can be done to prevent a similar occurrence in the future is to have a dedicated time before the flight to talk and brief the customer more in-depth on safety procedures. Additionally; on the final approach; I could have reminded the passenger once more to keep their feet on the ground and off the brakes throughout the entire landing and touchdown phase.