Regardless of whether the person is a Catholic or not, and regardless of whether the union is poltically active and supporting correct causes or not, the entire concept of a union is anti-biblical.
Collective bargaining is anti-biblical. See the parable of the workers in the vineyard Matthew 20:1-16. Any agreement over wages, working conditions etc is between the employer and the employee and does not concern any other employees.
Further a union operates through force and theft. It enforces it’s demands by stealing the owner’s use of their property (strikes blocking access etc). If the workers refuse to work the owner has every right to find someone who will work.
Unions derive their power through intimidation and the threat of violence. Against the employer and against any employee who gets out of line.
And they will make good on those threats.
Maybe not the good Christian union brothers. But they will look away while their buddies take care of business.