I'm not sure what you wanted to know about this?
If you don't want to get involved then just refuse to make a statement.
You will be a recent complaint witness. They do not add a huge amount to allegations really.
They can get any recent complaint from his medical notes anyway if he consents and if he does not then there are inferences to be drawn.
For what it is worth, I'm not sure I would get involved in an allegation like this.
It is possible they are interested in the data breach but that sounds more like a regulatory matter for the professional bodies.
However, if he has accused you of various indiscretions then I wouldn't get involved with him.
Can I clarify anything for you?