This is a great challenge attempt by Coughlin, not sure why they wouldn't let him challenge. It's strange to challenge that your QB DID fumble but if they got it back after the Ravens fumbled wouldn't it be first down for NYG? Great awareness by Coughlin though to realize that.
EDIT: They just said the ball would have been dead at the spot where Baltimore originally recovered, so it's not a play NYG should challenge. So in actuality, it was a poor job of awareness by Harbaugh for NOT challenging it. But hard to imagine any coach being that knowledgable of the rule there.