Colleague's work is consistently bad but they're a partner

I work with another dentist who's a partner in the practice. Their work is... not good. I've seen multiple cases where I would have done things very differently, and some that I think are actually substandard. Patients complain to me about their work sometimes. But they're a partner, so I can't really say anything. Do I have an ethical obligation to report this? Or is it not my problem? I'm conflicted.