My opinion has always been that if a supervisor is training an employee and between them they are only doing the work one fully trained seniority employee could do I don't have a problem it. You could grieve it if you want, but the supervisor in this instance isn't costing the union any jobs and I doubt the other employees on the belt want it off every 3 minutes as a 2 day hire can't keep up.
Now if you are talking the whole "I'm gonna come clean up your impossible to keep up work area every half hour and call it training game". The employee should really refuse the help and ask for a union employee to come help clean up the area.