In chapter 47, Edmund tells Fanny everything that has happened, and also about his meeting with Mary. During this he relates Mary's views:

“I will tell you everything, and then have done for ever. She saw it only as folly, and that folly stamped only by exposure. The want of common discretion, of caution: his going down to Richmond for the whole time of her being at Twickenham; her putting herself in the power of a servant; it was the detection, in short—oh, Fanny! it was the detection, not the offence, which she reprobated. It was the imprudence which had brought things to extremity, and obliged her brother to give up every dearer plan in order to fly with her.”

I find it very interesting how JA lets Edmund analyse Mary in this scene. Up to now Edmund was able to rationalize away all of Mary's clever remarks. But now he's really able see that Mary doesn't care about the act itself, she's only worried about getting caught in the act. Which is, philosophically speaking, quite a different idea.

I think he sees her for what she is for the first time and is devastated.