Co-supervisor comes to the office to help her students, which distracts me
I'm not sure if I would use the word "unprecedented" here - I have definitely had my supervisor come into my office to explain stuff (or just chat), and I have also done the same with my own students. I can't imagine that we are total outliers here.
That said, unless you have a particularly bad relationship to your fellow students and your co-supervisor, is there anything wrong with asking them to discuss in a meeting room (or their office) if these discussion are lengthy and/or happen very frequently? I certainly would not mind taking our discussion elsewhere if other people are bothered.
To directly address your proposals:
shall I message her
Why not just ask her friendly when they are discussing next time if they would mind moving to a meeting room?
mention her students
You can, but talking to both involved parties at the same time feels a little less passive-aggressive to me.
mention the issue to a superior academic or change office.
That sounds like a massive overreaction, especially if you haven't even mentioned anything to her directly so far. Reporting your (co-)supervisor to some higher authority is a nuclear action of sorts, which you should definitely not do without exploring other options first. And even if you report this, I have a seriously hard time seeing anything coming out of it (aside from a pissed co-supervisor, that is).
The more fundamental issue here may be that your expectation of a "productive" workplace may be different from other students in the room. If you go over to Workplace.SE, there are plenty of questions of workers trying to convince their co-workers to hush up - but the fact of the matter remains that a dead-silent, only-strictly-required-talking workspace is simply not the preferred option for many people (I actually find it extremely uncomfortable if I sit in a shared office where nobody ever chats).
The only real practical solution to this problem is either to change office, or get / order noise cancellation headphones.
Talk to your advisor about it, politely.
You can simply ask: would it be possible to to have regular discussions in a separate space? I find the noise a bit distracting.
There’s no need for petitions, HR complaints, rants to lab mates or other passive aggressive behavior. You are both adults, try to assume you’re dealing with a reasonable person before doing anything that may damage your relationship.