Don't forget that, like, it's a spectrum. On the scale of heresy, we have several options:
1. Things Dwarfs consider radical, but not unforgivable
2. Things a Dwarf might consider unforgivable applied to themselves, but other Dwarfs probably would not
3. Things that Dwarfs would consider unforgivable, but not the Empire (and perhaps not Belegar)
4. Things that the Empire would consider extremely controversial
5. Things that the Empire would consider unacceptable, but the Grey College might not
6. Things that the Grey College would consider unforgivable unless for a very good reason
7. Things that the Grey College would consider unforgivable period
Judging by Borek's reaction, we can probably safely rule out 1, and 2 is tenuous, but if something from 3-6 is at play, we probably want to learn about it and possibly derive some benefit for Order from it, be it by aiding Dum, having them provide info to us, coordinating against the next Everchosen, or whatever. And if it's 7, we also probably want to learn the specifics if possible, if only to have an estimate of when to expect the veteran Dwarf-equipped Shadowgave-led Beastmen invasion or whatever.