Since he has a job, your parents need to tell hiom what expenses he needs to be paying for himself, and they need to be encouraging him (should have been doing so for years) to save a portion of it for larger expenses. I personally think your mom should have hit the roof and told him that she will not tolerate him speaking to the man she loves like that, even though he is his dad. Sometimes the unexpected (which I assume that would be) can make the most impact.
As to why Dad didn't fix the air conditioner, simply say what it is--right now there is no money for that expense; it was either the AC or food for the table (or the college tuition/board, or whatever is most likely to give your brother a reality check).
