Earlier in the GR there was some discussion about whether Mary objected to Henry's plan to make a hole in Fanny's heart. I think the incident of the chain appears to places Mary deep in collusion with Henry in his project.

At the time of the chain incident, Mary had no idea that Henry would eventually declare himself in love with Fanny. Yet she seems to have been very willing to try to persuade Fanny to select the chain that was from Henry. This seems to make her an accessory to Henry's plan and sinks her as low as Henry.