Divorce records are public. Go to the county Clerk's office and read through all the documents (Divorce Decree, Property Settlement;Settlement, etc). It'll be depressing, but that'll show how much your father is sending to your mother.
Note that both sides might be telling the truth. Depending on where you live, housing expenses can be shockingly high, and teens -- through understandable lack of experience in the world -- usually don't have a good grasp about how expensive stuff really is.
Thus, two houses -- and therefore two mortgages -- might just be eating a lot of that money. Add on to that any possible poor spending habits by either side and POOF there goes the money that would have been spent on college.