If you truly believe that it takes all kinds then you have to consider that this man may just be sufficiently spineless or devoted to his mother that he has chosen to not grow up (that is, I don't buy the cheating or gay explanations of another commentator).  Letter writer may believe that she's doing the man some good but she's probably just enabling his behaviour.  Dave is certainly right--moving on is the only sane course of action.  Dave's bluntness is really kindness.  There's no other sane choice.