An individual can be physically MMI and yet not be psychologically MMI.
If it is PTSD, it is possible that the patient is physically recovered but cannot return to the environment in which the injury/trauma occurred.
But assuming that this was not PTSD, but was depression (mood) or a pain disorder (somatoform), it is less likely that there is a psychological disability after reaching physical MMI.
Additionally, returning to work is certainly more therapeutic than remaining at home, napping, eating, smoking and watching television. Work itself is a healing process. The individual has human contact, is productive and is less dependent upon others.
Your first task is to determine what psychological disorder is purportedly disabling the patient. Secondly, how does the individual clinically benefit from not being in the workforce since the psychologist is blocking him from doing so? (This is rarely the case) And finally, are there other incentives to remaining out of work (settlement considerations, financial support by wife, enjoyment derived from having minimal responsibility, and/or not having a rewarding job to which to return.)
What can happen is this: The patient is physically released MMI with a specified PPD rating. There is no work for which they are trained/educated within those permanent limitations. They remain depressed, and the depression immobilizes them. Even in this case, however, the solution is not psychological treatment that keeps the patient dependent and fails to return the individual to some form of productivity.