There are two levels on which to approach this: personal and task-level.
For the next task you are going to do, ask the person what success looks like. In other words, what would you have to do to correctly complete the task. Sometimes just the act of saying it out loud is enough and reducing the possibility for subjective criticizing later. It also gives the other person a more concrete appreciation of what you are doing.
The other approach (you should probably do both) is at the personal level. Nobody wakes up in the morning, looks at themselves in the mirror, and says "I'm going to be a bad person today." No, people are mostly are just trying to pursue their path to happiness. The problem is how skillful they are at finding a good path and judging what is helpful and what is not. It is important to recognize that this other person has hopes and dreams, probably wants to be a good person, probably even thinks he or she is being a good person. When you humanize the other person, you can start to feel for how rough it must be for them to have gotten to this point, to be making the decisions that they are making. Optimally, figure out what is motivating them, but, at the very least, attempt to connect at a personal level.
Not knowing the specifics of your situation, I hope this helps.