There's a few things a union can do in this situation. What they can't do is prevent blowback from the professor -- not even the university has leverage there.
The resources a union have typically amount to (a) a grievance process, (b) work stoppage, and (c) a pittance of a legal fund. Work stoppages don't occur because one professor mistreats one student -- there's no rule against that, but unions are democratic and that motion would simply never pass unless the harm was particularly egregious and the victim was especially charismatic. Politics be. A grievance could "succeed" but ultimately they're pretty toothless -- the university has little leverage over a prof, the union has less. So we're left with a pitiful legal fund -- know how everybody hates dues? Unions are weak as a result. The student could potentially sue their prof through the ordinary legal system. They could be awarded monetary damages, but they would not be awarded a degree.
The professor is the judge, jury, and executioner for the doctoral thesis. Other professors in the department will be working alongside the offending professor for the rest of their lives so whatever politics be in the union, they're way more intense in a department of professors. And even if you do find a sympathetic ear with another professor in the same department, they're probably not in your specialty, because when there are two profs with the same specialty in a single department, one hired the other and they're bosom buds. And this is the real reason that individual grad students don't speak out, file grievances, etc. against professors, and known rapists carry on for decades: to act is to throw away your degree, and with it, your entire career.
The resources a union have typically amount to (a) a grievance process, (b) work stoppage, and (c) a pittance of a legal fund. Work stoppages don't occur because one professor mistreats one student -- there's no rule against that, but unions are democratic and that motion would simply never pass unless the harm was particularly egregious and the victim was especially charismatic. Politics be. A grievance could "succeed" but ultimately they're pretty toothless -- the university has little leverage over a prof, the union has less. So we're left with a pitiful legal fund -- know how everybody hates dues? Unions are weak as a result. The student could potentially sue their prof through the ordinary legal system. They could be awarded monetary damages, but they would not be awarded a degree.
The professor is the judge, jury, and executioner for the doctoral thesis. Other professors in the department will be working alongside the offending professor for the rest of their lives so whatever politics be in the union, they're way more intense in a department of professors. And even if you do find a sympathetic ear with another professor in the same department, they're probably not in your specialty, because when there are two profs with the same specialty in a single department, one hired the other and they're bosom buds. And this is the real reason that individual grad students don't speak out, file grievances, etc. against professors, and known rapists carry on for decades: to act is to throw away your degree, and with it, your entire career.