Has it ever been beneficial to let a co-worker fail to make a point?

I’m in a bit of a moral dilemma. I have a co-worker that is on paid time off for a few days. There was an agreement between us that I would be the one to cover their job duties. Largely, because there are certain people that she doesn’t want to do it. Come to find out that the person she really didn’t want anywhere near her stuff, is the one doing it. I have been trained by the co-worker on PTO to perform the task exactly as she wants it done. I know for a fact that it will not get done that way. Trust me on this.

This person isn’t my supervisor, but a supervisor, nonetheless. So, there is absolutely nothing left for me to do. I let her know of the agreement made, but it made no difference.
I have been told by other co-workers and my supervisor actually, to just let her fail with the hopes that she will not be allowed to do the job again. When I say fail, I’m using it in the sense that the job won’t be done to the standards that it needs to be, meaning “half-assed”.

Sorry for the drama, but I started wondering if that actually works. I’ve never been able to sit still long enough and let it get that far. I want to get the job done and move on with my day. Has anyone ever had any experience with letting a co-worker fail?