D.N.
I am the mother of 2 whiners. My son will whine if no one seems to want to play with him and one of my girls will whine if all the others do not want to do what she wants. If the whiner's mom is looking to you to correct your child, then she is not helping her own. When my son whines that he has nothing to do because the others do not want to play, I tell him he either needs to go ask what the others are doing or actually ask if he can help/play, etc. When my daughter complains that the others won't play with her, I ask questions and then tell her she needs to understand that not everyone wants to do the same things and they need to come up with some kind of compromise. If this doesn't work, they can sit it out. They usually don't sit out very long. Granted, there have been times the others were just being mean because they knew what the result would be. Then I have stepped in to all-though I have gotten some looks from other parents. If you are close to the child, maybe you can say something to him. Otherwise, you will need to talk to the mom.