There was an earlier discussion about some characters saying it was all Fanny's fault because she didn't accept Henry's proposal.

But I suppose you could also say that it was Mary's fault. Her conniving got Henry and Maria into the mess, because even though she knew how jealous Maria was, knew how Henry felt for Fanny, she still managed to arrange for Henry to come to the party and meet Maria, out of "a little curiosity" as she writes to Fanny.