To be fair, I've heard the "you can't give a bad reference" from many sources.There’s nothing in law that entitles anyone to a good reference. If the union is saying otherwise, I wouldn’t trust any other advice they give.
Nobody likes to be called out on their performance but don’t automatically rule out that there may be an issue. As others have suggested, your partner should engage with the process but ensure everything is in writing. Ask questions, look for clarity etc. Convey the sense that she wants to turn things around.
She’s naturally hurt and her back is up. It may be that the manager just isn’t handling the process well. They may be relying on a gut feeling that performance isn’t adequate but incapable of articulating the specifics of what the problem is or how it can corrected.
IMO it's a case of Chinese whispers. The message started with you can only state dates title and nothing else, to you can't give a bad reference, to you can only give a good reference.
Most of the big companies only provide dates / title. I've had managers ringing me off the record (I can't do off the record as I'm HSE).
It's fine if they tell me the ee was exceptional etc., but once I tell them I can't do "off the record" and they end the call, I know enough to make a decision. Usually I can't do anything.
In 20 years I haven't proceeded to contract with about 10. If I could use that information it would be in the 100s.
I issue about 500 to 1000 contracts a year, depending on what Paul R says. .