Well, Tokiomi went into a lot more detail in the anime than in the original light novel. In the light novel, Kariya had absolutely no reason to trust him. In the anime, Tokiomi's talk should really have given him reason to at least mention Sakura's treatment.
Having said that, if he had done so, Tokiomi might have accepted it as a "necessary part" of becoming a magus (and assumed Kariya's objections were due to him not accepting magecraft). Further, even if Tokiomi had discovered the truth and realised what an idiot he'd been, I'm not sure he could have actually done much about it.
The whole thing is a huge tragedy. Both Tokiomi and Kariya were acting in what they saw as the best interests of Sakura (and Rin), but neither one of them were able to look at the whole picture. Tokiomi only ever thought of Sakura's development as a magus, and didn't consider what it would mean for her as a person, whereas Kariya never considered the magus side of her life, and just thought what was best for her as a normal human.