For some reason I am imagining this as either an investigation into the allegations of Meghan Markle’s bullying at Buckingham Palace or the Red Bull Racing investigation into Christian Horner and whatever he was accused of.
Let’s say you are friends with either Christian Horner or his PA, the HR idiots are Red Bull’s HR department, and there is third party (Barrister) investigating.
Do you have the option of communicating directly with the Third Party? If so, I’d suggest doing that as much as possible, copying in your friend and HR if appropriate.
Another option is cutting out HR completely and going straight to Third Party. Given that you don’t work for Red Bull, it would be inappropriate for your information/testimony/evidence/whatever to go through them as it may impact the impartiality of the investigation and, of course, HR wouldn’t want that, would they? (You picked up in the sarcasm there, right?) If you do that make sure you also exclude your friend, or just send stuff to his non-work email address.
As for one liners to shut down HR:
As I am not employed by Red Bull, it would be inappropriate for me to (do whatever they are asking).
Optional extra - add the following to the sentence above - but I shall ensure that information reached Third Party.
For your information, I am not employed by Red Bull (yeah, they already know but this is a polite, ok, semi-polite reminder) which makes the tone of these emails demanding rather than requesting.
Are you asking me or are you telling me to….?