...Edward had mentioned anything about her when she was still staying at Norland.

He was staying with Lucy's family for two weeks. I'm more inclined to think that the tone of his letters had cooled towards her while he stayed at Norland, and when he arrived and was in such obviously low spirits, she began to work on him right away to find out the particulars. She may well have already had a suspicion that it involved some woman he had met.