Hi Suzanne,
That note was included in the text at Susan Wynn's behest, who heard it from Dr. Xie of Chi Institute fame. It doesn't appear to be based on any scientific research. Indeed, if you look up Bupleurum in the medical literature (e.g. pubmed), you'll find the opposite - namely 30 or 40 studies lauding its benefits in liver disease.
Myself, I would have to side with the scientific literature. I've never seen it cause liver disease. Any formula can aggravate liver disease, I believe, if it induces the wrong hemodynamic (e.g. brings blood to an acutely inflammed liver). But apart from this general impact which any formula could have, we haven't seen the slightest potential for problem with the use of Xiao Chai Hu Tang in liver disease.
Hope that answers your question.
Steve