BE-55 Instructor Pilot reported while the student was attempting to exit the runway they were too fast; resulting in a runway excursion.

Narrative

We were conducting FAR Part 61 flight training in a Beechcraft Baron E55. The student is a commercial pilot (ASEL) with an instrument airplane rating that is training for commercial pilot multiengine add-on. They also possess a flight instructor airplane certificate.We were doing pattern work when the event occurred. We were instructed to turn left base over the Location A just north of ZZZ; which we did. There was traffic on right base to runway XX that was #1 for the runway as directed by tower. We were #2 for runway XX and cleared to land behind the traffic. After touchdown during the rollout; the student began drifting to the right of centerline. As we were approaching taxiway 1; the tower instructed us to exit at taxiway 2 and taxi to parking via 3. As we neared taxiway 1; my student attempted to take the turn at a speed that was too fast to make the turn. I advised them to brake because the instructor-side on this plane is not equipped with brakes. Just before taxiway 1; I urged them once more to brake more; at which point I declared 'my controls'. I attempted to bring the airplane back towards centerline to avoid going into the grass; but could not at this point. We ran off the runway about 10-15 yards just on the south side of taxiway 1.One of the contributing factors to this event was tower instructing us to exit at taxiway 1. We should have advised unable and continued down the runway to the next available exit. As PIC; I should have advised the student of this earlier during the rollout and could've prevented the event altogether.Another contributing factor is not having brakes on the instructor-side of the aircraft. With brakes on my side; I could have aided in the braking of the aircraft on the runway and potentially have avoided the event after taking controls.Finally; as PIC; I should have considered taking the controls earlier during the rollout to have more time to correct the student's offset to the right of centerline.