This question relates to UK employment. In addition, it does not concern me, I am asking on behalf of someone else.
The person in question has had an up and down relationship with their manager for a while but overall things have been working well enough. However a recent decision by the manager seemed rather unreasonable and was but with no real explanation or justification given. The decision was questioned because the employee has extensive experience on the subject matter and the manager does not, and was undertaken in a sensitive fashion. However, the manager has been quite intransigent about it, even going so far as to threaten that they would "talk to HR" (although about what exactly wasn't clear). Attempts by the employee to do the task assigned in a manner consistent with the quality and care that they expect from themselves and which would not be unreasonably expected by an employer, have been rebuffed (basically, doing some work on their own time, voluntarily and with no expectation of reward).
The employee would really like to sort this out in an informal manner and has been exploring the possibility of mediation which was recommended by ACAS, but it isn't clear what rights, if any, the employee has to request it or if there is any onus on an employer to go down this route if requested. This is particularly pressing since the manager is now talking about a performance improvement plan, which seems rather unjustified since previous appraisals have all been graded at a level where there is only one better mark possible. This is rather a blunt instrument to wield so freely and it is starting to look more like a personal witch hunt (for reference, I found this article on the Guardian).
What rights, if any, do employees have to request mediation when the relationship and trust between themselves and their manager has broken down? And are employers in any way obliged to agree to mediation before resorting to heavy-handed actions like PIPs?