Other than more carefully noticing, “hmm this data looks like a polynomial curve, maybe it’s fabricated?” I’m not sure what you propose the professor do instead.
The professor is not going to duplicate all of the experimental measurements done by the students and it seems totally natural to me to trust the student is not committing fraud, until there is evidence otherwise.
Taking the professor at the word that it's those graduate students' fault (who very conveniently can't defend themselves and haven't been at the University since 2020) and not the fault of the professor is a bit strange. It might after all be their fault, but it could have easily been some manipulation done or demanded by the PI who wanted a publication.
What I'm trying to say is, I'm not willing to take the professor at their word here when their word is so extremely convenient for them.
Duplicating everything is unnecessary. Duplicating the thing which is foundational to a groundbreaking discovery is necessary. Beyond fraud there could be just an honest mistake or some fluke confounding factor.
Maybe there should be a "reproducability risk" metric, calculated with the transitive closure of all published results that reference a paper that has yet to be reproduced. This could help researchers calibrate how much they should trust a result and indicate when a foundational paper really needs to be reproduced.
Duplication of work prior to publication is standard in many experimental fields, like synthetic chemistry. In some cases the cost of an experiment is high enough to be a problem, but conductivity measurements are not really that exotic.
The professor is not going to duplicate all of the experimental measurements done by the students and it seems totally natural to me to trust the student is not committing fraud, until there is evidence otherwise.