Yes, milk thistle has been shown to be very helpful for dogs with liver problems. I have two or three articles about it, if you'd like to read them.
One of my dogs is being looked into for liver problems (we're going in Saturday for more tests). He's on milk thistle, an antibitoic, SAMe and a mild digestive enzyme.
Doberkim is right, though, some supplements are better than others depending on the exact problem.
What are you feeding CJ?