Graham McDonald and Carmella's Family Crisis

Investigators left Canada for sunny Florida to ask Matt. They went to his house, they left a business card and spoke to him on the phone. He said that Heather's a liar. With no jurisdiction in the US or any evidence to press Matt further, investigators came home empty-handed. But then they did some digging. Graham McDonald was also friends with Matt's brother David. So why had Matt rushed out to see Graham in the middle of a family crisis and not his brother?