Although withholding information from someone who would find that information painful is sometimes justified, there is no such justification if the person would benefit from having the information.
Conclusion
Jane should inform Jason that his supervisor is displeased with his work.
Evidence
While it may be painful for Jason to learn his supervisor is displeased, having this information would allow him to improve his supervisor's opinion.
Withholding information is unjustified if the person would benefit from having it.
Evaluate
The argument essentially states that while it might hurt Jason to find out about his supervisor's displeasure, causing him pain is justified because he would benefit from the knowledge by being able to improve his work.
We're looking to bridge the gap between "knowing the information allows improvement" and "improvement is beneficial." Why should Jason be informed if it's going to hurt him? Because he can improve his supervisor's opinion, which is assumed to be advantageous for Jason.
Goal
Look for an assumption indicating that the benefit of knowing outweighs the pain or establishing that improving his supervisor's opinion would indeed benefit Jason.