I just can't see this as anything but the sort of coincidence that happens in novels to further the plot... It may be unlikely in RL but we accept it. I can't see how Lucy woudl know when or if Mrs J was likely to be in Exeter or unless Edward had been talking of Mrs J and how friendly and hospitable she was, how she'd know if she had much chance of getting an invite. I think it is just dumb luck.

She was visiting Exeter, planning to see soem friends and happened to Meet Mrs J. They fell into conversation and Lucy managed to find soem distant connextion between them and cadged an invite. I think the fact that Mrs J knows the Dashwoods is a bonus, but L would problaby try and finagle an invite anyway, as Mrs J's a rich old lady and good natured.. Im sure that Lucy woudl throw over or postpone whatever engagements she had, to get a free couple of weeks with Mrs J/Sir John. That's how she and Anne made their way in the world, finding richer friends and relatives who will invite them for visits...