I think putting the law firm's name and address at the letter would be much better. Also, you've got something important you must do now. With your knowlege, you must report the abuse and horrible things going on there. The other clients, future clients and most importantly, the animals, must be helped. He should be put out of business. Sounds like a horrible place to work. And yes, if it were me, I can truly say I would report the abuse and do everything I could to prevent any futher bad things for happening.