May a union representative attend the harassment investigation interview?

We are preparing for an investigation interview regarding a workplace harassment complaint, and the complainant has requested to attend the interview accompanied by a union representative.
Until now, interviews have been conducted only between the investigator and the complainant, so this is the first time I have received a request to have a union official present.
Since the intention is that they find it difficult to talk alone due to psychological instability, I don't think there is any reason to prevent them from sitting together at all.
On the other hand, I am concerned that having a third party present during the investigation might influence the statement or leak the details to the outside.
If you allow sitting together
It is unclear whether union officials must literally just stay by the side, or if they are allowed to offer opinions or ask questions on behalf of the union in the middle.
Are there actually any companies that allow union officials or employees to accompany staff during investigation interviews?
If you allow it, I would like to know if you provide separate confidentiality notices before the meeting or if you also determine in advance the scope of intervention by the accompanying person.