How do I say this to my colleague

[StevebutnotJobs]

I've been in a jobshare for a year now, I'd been trying to go part time with my employer for sometime when this role came up. Sorry this will be a long one.
The jobshare isn't working as my jobshare partner isn't pulling their weight, is missing things that need doing, failing to hand things over, making mistakes and making lots of excuses. Up until now this mainly only affected me as I spend lots of my time following up on things she hasn't done or putting her mistakes right where I can.
Our manager is fully aware of this as I have spoken to her numerous times about it and recently a complaint was put in about my jobshare partner failing to do something last year which has only recently come to light but has consequences now.
A colleague has just come back from Mat leave, we were her cover and another mistake has been found that my jobshare Partner has made and also previously made. We will also be her holiday cover going forward. I can cope with mistakes as long as I know about them but jobshare partner will not take responsibility for anything just gives lots of excuses then doesn't follow through.
I spoke to my manager again to see if I could go part time rather than jobshare but she is not keen due to this affecting the business. She suggested the 3 of us getting into a room and hashing it out which i'm not keen on as I don't want to jump all over my colleague when she doesn't know what's coming. Plus I have tried in quite a few nice ways to get her to up her game.
However she has been at the company for 12 years and has lots of gripes about the way things are now. My manager says she was probably elevated when she shouldn't have been and says she has suggested getting a mentor and using the company onboarding system to get refresher training. Neither of which my jobshare partner has done.
My manager is now saying that as I have first hand knowledge I should speak to Jobshare partner and lay it all out for her that things aren't working as they should be and provide evidence and then manager will speak to her separately. She also pointed out that I may do things my jobshare partner doesn't like.
I actually get on well with my jobshare partner and would like that to continue. I am not senior to her at all and don't think I should be acting like her manager with this. It def. needs to be done though.
How have other people dealt with this kind of situation?

Your manager isn't managing her team

I would ask for a round the table discussion, with your job share and your manager to discuss how the share is going

You need to air your issues to your job share, but also be prepared for her to have some issues too and take them on board

@PickledPurplePickle i'm happy to take anything on board

To be honest, this sounds like poor management, as though your manager doesn't want to deal with it so is letting everyone 'hash it out between them'. If someone is underperforming, the manager needs to deal with it one-on-one and document it.

Perhaps you can say that yes, you will agree to be in a room together but only if HR (or a neutral third party) is present and notes are taken.

Don't do it, you are not a manager, you are not paid to manage staff and if you shout down with your colleague telling her of the issues her mistakes are causing it will damage your relationship for ever. Your manager needs to step up and do their job, make it clear to them that it's their responsibility and if they don't follow through you may need to go above them.

@greenplantsandtrees I don't think we are at the stage of HR needing to be there, that's further down the line surely

Honestly, I'd tell your manager no. It sounds like they're trying to take the easy way out and avoid dealing with known performance issues. It's not your responsibility to fix those for your manager. If previous steps implemented by the manager haven't worked, then it's on the manager to take the next appropriate steps. However, I do think the idea of getting together with your colleague and manager is something that might help. Unless your manager has detailed knowledge of the tasks you do and how they're done in practice, it's possible something is being lost in translation. If your manager handles the messaging right, it should come across as supportive. Perhaps you could go back to your manager and say you'd be open to meeting as a threesome provided you and manager agreed some strategies beforehand and you agreed some ground rules for the meeting (which your colleague should also contribute to setting).