I dont know what your wife's degree is in but she may like the feeling of being in control in caregiver positions. Or she might also have some codependent type caregiver traits. No advice other than I would definitely encourage her job search through the "we need the financial stimulus" approach.
If you tell her she would benefit from new friends or from the mind stimulation, she might take it to mean she is dull, boring and needs new friends... . I have learned in my situation that it is never a good idea to suggest that uBPDh and/or our family could benefit from changes on his part!