] Suddenly Edward is practically penniless. That could make anyone feel desperate.

This is apparently the type of 'desperation' that made Willoughby act, or at least people would assume so if the news is all over town that he is "all to pieces" as Mrs. Jennings has reported.

I guess this is another reason the colonel would respect Edward's behaviour, because it is in such sharp contrast to Willoughby's. Edward, suddenly deprived of his potential inheritance, stands by the penniless girl whose affection he has (apparently) secured, whereas Willoughby abandons two girls without inheritances when his inheritance is revoked.