If that's the professor's scientific judgment, then there is no reasonable way to challenge it. If the supervisor doesn't support a thesis, then it's going to be hard to convince a PhD committee of its merits.
Before doing anything else, that PhD student needs to find a new supervisor ASAP.
Thats not really true everywhere. My ex-gf basically pressured her professor to fianally give up on critiquing her rather unscientific way of writing, and give her the PhD because he was basically afraid of what a woman with a disability could do to his reputation.
Was a rather traumatic experience for me, because I had to watch and learn that a degree these days isn't really worth much.
Hahaa, separation can do that, right. But in this case, I was only refering to me having to watch as someone uses their gender and disability to gain unfair advantages...
It's far from trivial, but certainly possible to challenge this. If the student has produced research that would be considered worthy of a PhD by most experts, they have a chance to still get their PhD unless the local university politics are extremely dysfunctional.
It is not for thesis professor to decide graduation as it done by committee – in theory professor input does not play big role when defense starts. There are lengthly procedure with personal reviewers for thesis which dissects it through and through. Although on hand is hard to judge form one sided information, it can be just professor want to have extra published papers, the scientific worth is questionable (i saw few times - thesis is like rewritten book) or just academic nepotism starting show signs.
Before doing anything else, that PhD student needs to find a new supervisor ASAP.