This!!
I’m a branch secretary for a large union. If your husband really feels he is being singled out and is already part of a union, tell him to contact them now for support.
If a manger has concerns about an employee they should be communicating them clearly and setting out a clear plan for improvement. They should also take into consideration whether there are any health issues that may require a reasonable adjustment to be put into place, or whether the member of staff is stressed and this is impacting their ability to do their job. If this isn’t happening I would send an email to the boss asking her to clearly outline the issues she has identified and how she proposes he makes improvements. Make it clear that he wants to improve and would like support and direction to be able to do this.
It sounds like quite a small company and in my experience they can be pretty lax about having any set or written procedures in place for dealing with thing like capability/performance issues, but as a minimum I would expect that if meetings are being held to discuss his performance then he should be able to have a union rep or colleague present. This would help to prevent any potential gaslighting that may be going on. I would also expect that the manager should put any concerns discussed in the meeting in writing, so your husband can refer back to this, and log the changes he is making to address the concerns.
If any new work practices or procedures are conveyed to your husband verbally, I’d tell him to follow up with an email. It can be laborious and irritating, but having an audit trail is so handy if things don’t improve and a grievance needs to be submitted. Just something along the lines of ‘ just to confirm that when we spoke earlier it was agreed we would do things x way. In light of recent events I just want to make sure my understanding of the new procedure/practice is correct. No need to respond to this email, if it is and we’re on the same page.’
If any training needs have been identified, make sure he follows up on this and requests it in writing, but if things don’t improve in a few weeks and he has sufficient evidence to support his claim, I would encourage him to submit an informal grievance on the grounds that she is managing the situation poorly, and is failing to set realistic expectations or communicate clear instructions and provide support and direction.