DFW-MIA? Wouldn't that be mainline (as opposed to American Eagle/Connection)? I was under the impression that airlines don't usually do gate-checked bags for mainline flights - it's only for Regional Jets.

I know I've heard announcements onboard a mainline flight, just prior to departure, as they try to identify the folks who left their suitcase at the end of the jetbridge, thinking there was gate-checked service. Did your friend perhaps leave his/her suitcase at the end of the jetbridge assuming that the suitcase would be whisked away to the bottom of the plane, when nobody was looking to do so?