She should file a SLAPP lawsuit against the union.
No need to file a separate law suit, probably. Typically Anti-SLAPP is done by a motion early in the case.
Don’t you mean an anti-slapp lawsuit?
It’s still a little confusing to me, but the rule is to keep a larger entity, like the school board, from using the courts to punish an individual for exercising their rights. Slapp stands for “strategic lawsuit against public participation.” That is what the union was threatening to do: file a baseless lawsuit that would not succeed in doing anything other than bankrupting the mother.
Hence it’s the mother who would have the anti-slapp option, but only if they pull the trigger on their threat first. I don’t think you can file a lawsuit against a threat.