I agree with you.
All Hastert has to do is stand up and state that he did not have the full story when he was first informed and, had he had the full story (the IMs etc.), he would have immediately advised Foley to resign or face the consequences.
End of story.
Hastert has issued a statement (today, concerning investigation):
http://releases.usnewswire.com/printing.asp?id=73543