We learn from Sir John that Willoughby is a fine man to be had, so I can completely believe that Miss Grey was one of his admirers the last time he was in town. We can't discuss the particulars of their engagement at this point in the book, but just a look at his behavior towards the Dashwood sisters, and Miss Grey's look of steel at Willoughby, it seems very likely that she knows all about Eliza II, and has basically blackmailed him with her fifty-thousand-pounds.

Well done, Miss Grey. You seem to have claimed your prize, and fitted it up in a nice, tidy box all at once.

One could almost feel sorry for Willoughby at that point.