Firstly it is not the same as a written warning. It won't be recorded as such on her file, and a formal disciplinary process won't have been used, which means it won't be a formal written warning on her file for a set period of time, 6 months or a year. With a formal warning, if she did anything else during that time, they could jump to the next stage of the disciplinary process, which could risk her job. Also it could be disclosed in a reference.
A meeting with a manager with a note made is not remotely the same thing and doesn't have nearly the same implications for Jill. If Jill insists on bringing a union rep that steps the whole thing up and your manager may insist on it becoming a proper formal process and a formal warning being given.
Also, your job as manager is to generally support and implement what your own manager does and directs, not to undermine it, which you would be doing here.
I manage a small team. Janet and Jill have a difficult relationship. Jill is my deputy manager, Janet is a more junior member of the team, but a very strong personality.
Janet was extremely aggressive towards Jill, who submitted a grievance, leading to Janet getting a written warning. (I think she should have been dismissed, due to the level of aggression).
Several months later, Jill let slip some personal info about Janet to another team member, who spread it around.
Janet wants Jill to get a written warning - a bit tit for tat I think. My understanding is that Jill should apologise to Jill, as should the other team member. Janet wants nothing to happen to the other person, only for Jill to be disciplined. She is so insistant about this that she went to my manager about it.
My manager wants to have an informal meeting with Jill, and for this to be minuted, and have this put on Jill's HR file, but says this wont be a written warning.
I already have it recorded on Jill's supervision notes, have given firm management guidance, and feel that my manager is offering this 'on file' thing as a sop to Janet, but I think it is, in effect, so similar to a written warning that Jill should have a union rep at the meeting with her, to protect her interests.
The advice I'm asking, is whether to inform Jill of what is about to happen, and suggest she gets a union rep for the meeting?