She is a corporate finance lawyer, not a libel lawyer. And lawyers do bring cases they have no chance of winning. They are often too close to things to take an objective view and reject independent advice.
Remember that, when this row first blew up, she sued the charity and reported it to the Charity Commission for "poor governance, weak executive management, abuse of power, bullying, harassment, misogyny, misogynoir – and the coverup that ensued". The Charity Commission found no evidence of "widespread or systemic bullying or harassment, including misogyny or misogynoir" or "over-reach" by either Chandauka or Prince Harry but acknowledged a "strong perception of ill treatment felt by a number of parties".
My personal view is that her public statements when she reported Sentebale to the Charity Commission were far more damaging to the charity than anything Harry said. His statement at the time (issued jointly with Prince Seeiso, the co-founder) was that the relationship between the trustees and Chandauka had broken down beyond repair, creating an untenable situation. He said that they asked Chandauka to step down, but her decision to take legal action to retain her position underscored the broken relationship. The statement went on to say that they would be sharing their concerns with the Charity Commission. All the other trustees resigned at the same time.
I note that Sentebale is only suing Harry and one of the trustees who resigned. I wasn't aware that they had said any more about it than I have stated above. Obviously we don't know the details of Sentebale's case as yet, but if it is based on the statements that Harry made at the time, I think they will fail. Both the honest opinion and public interest defences appear to apply and give Harry and Dyer complete protection. However, my view could change when we know the details.