Focus on what the querent wants to ask, even if it means we can't answer it here
When we edit an open question we should always be improving the question that OP asked, but we should not be changing the core of the question. Not, at least without explicit confirmation from OP that the changes are agreeable to them. This means that sometimes there is nothing we can do to "fix" a question in the case that it is clearly POB and OP either does not want to change it or doesn't respond. And that is fine. If someone else has the issue, they can ask it in a way that they think will not be POB.
Close POB questions first and then work with OP if there might be a way to change it
When a question has issues that affect its ability to be answered well, one of the first things we should do is work on getting the question closed so the issues can be worked on. If a question is clearly POB, this should be our first step always.
If someone sees a way to change the question such that it shifts the question a significant amount from what OP was originally asking there are two way to go about this and both involve working with OP before reopening the question:
1. Work with OP in the comments to change the question
If you see a way that a POB question can be salvaged and it involves a potentially significant shift in the focus of the question, pitch the change to OP in the comments and wait for them to respond. If they agree, then someone can make that edit and vote to reopen. If they decline (or don't respond), then we just leave the POB question closed.
The downside to this method is that pitching changes to OP in the comments can be very difficult especially if there is help piling going on and the OP could be overwhelmed and/or confused about what is going on.
2. With the question closed, make the edit you propose and then leave a comment explaining it and asking OP for approval
Another way I've seen this done is to make an edit to a closed question and then ask in a comment if OP agrees with the significant shift in the question. If they agree, reopen the question. If they don't or don't respond, the question should remain closed.
We should never be editing an open question to change it significantly from what OP is asking without their consent. If we did this method to an open question (or open the question without waiting for OP's consent) then answers will start rolling in before OP has a chance to even comment on the edit. This is not something that should happen and ends up potentially not being good for answerers (if changes to the question have to be made later eg) or OP (if changes to the question are not to their liking).
The downsides here are that editing a closed question into a good-looking question often does result in getting the question reopened through the review queue or people passing by that may not realize what work is being done. If you see this happening, I think flagging the question for mod attention would be appropriate. A mod can unilaterally close the question again until we get a response from them.
If, at any point OP declines the changes or if they never respond we should leave the POB question closed. If someone else has the same issue that they think they can ask in a way that is not POB, they can freely do so. However, questions should always ask what the OP intends them to ask, even if it means the question cannot be answered here.