I am not one for a toothbrush. In fact, I hate them, as do my humans when they are chasing me around the room trying to stick that non-edible thing in my mouth. So, the next best alternative, teeth cleaning treats. I have a few favorites but I love, love my greenies. I can chew through them pretty fast which might be defeating the purpose, but maybe your friend is not as food-driven as I am. Rawhide treats can also help with reducing plaque and build up and they are a bit tougher to work through, If you dog chews through the teeth treats and rawhides too quickly, consider a Kong treat, they can reduce build up as well.
While there is no substitute for teeth brushing, one of my favorite treats I mentioned might help to keep the bad stuff down in between chasing us around with the brush.