Trust me, CH would NOT want this going to an Employment Tribunal. Although evidence docs are usually kept private, evidence can be detailed in 'reasons' and 'judgments' e.g. public documents. Anyone can find those, and it is quite rare in my experience that a tribunal can be fully gagged. The docs could be anonymised or pseudonymised (vague details such as Claimant A and Respondent B) could be used. However, it wouldn't take long for someone to piece the facts together and confirm whose case it is, especially if obvious details are left in (and they often are).
It's appalling to me how the poor woman seems to be isolated, and put into a corner here. If she feels she has no other choice but to quit her job, she would also have a constructive dismissal claim - and whilst that's hard to prove, it bumps up the tribunal and means more evidence can be considered.
She needs to get the support she needs in this matter. If it's ok for Horny to swan about like the absolute twit that he is, she should also be able to provide vague comments. But instead, she's being silenced in the hope that she drops it and walks away. It's terrible legal and HR practice and can be seriously damaging long term.
If I were advising her, I'd also be telling her to look into personal injury on account that her mental health has been battered by this whole process - personal injury is through the high court, so even if she signs any settlement agreement she should still be able to take action through the high court if she's been negatively affected i.e. mental health conditions have developed or relapsed etc.