That you're using the word "confront" suggests that you expect the manager to be defensive. If you approach with that expectation, it's likely to happen. Do you have any evidence that the manager reacts defensively to the kind of advice you're suggesting? While it's possible that the manager may not be receptive, it's also possible that s/he will. What evidence do you have either way?
If a manager is defensive to feedback and suggestion, that's a different problem to the one you're trying to address, and I would suggest working on that first. If someone is defensive to feedback, this is the most important feedback to give them, especially for people in management. It's far more risky IMO to have a manager that can't take feedback than to have one with optimism about risk, since the first will lead to the second plus a whole load of other problems.
Otherwise, you can use standard feedback techniques - anchor the things you value about the manager; show the evidence for your concern, including any previous experience you have; explain the impact that makes it your business; and above all, treat it as a journey to the place you want to get to, rather than an argument in which you've already taken one side of a confrontation.