Yes, you can do this. You just send a letter asking to withdraw the application. It'll take some time, I understand, to get a confirmation it's been withdrawn. I don't know what gets refunded.
But: if you think she will want to come in any period up to eg two years from now, I think withdrawing is a mistake. It makes more sense to simply let the process go forward, and when she gets the COPR / visa, she decides when to come and 'land.' After landing, she is a PR.
But at that point, she doesn't have to stay if she's not ready. She can leave and return basically at will (once she gets her PR card anyway). The only wrinkle is that she must spend 730 days in the first five years (from the date of landing) to be compliant with the residency obligation. That's why i say if she returns to Canada within roughly two years from the day of landing, she should be okay and will have a buffer in which to become compliant. (If she ends up out of compliance, there are issues that start to arise - but it's not an immediate loss-of-PR status, there's much more to it)
So there is no major advantage to cancelling at this point. If you thought she absolutely would not be able to return to Canada in the first three or four years or more - yes, then withdrawing would probably make sense. But the residency obligation requirements are sufficiently light that they allow new PRs to take a bit of time to return and settle.