I had overlooked that passage, (it's in ch.36 btw). Thanks for pointing it out. Mary answers Fanny at some length and claims that Henry's habit of meaningless flirtation is his "only fault". She simply does not understand Fanny well enough to realise how serious a fault this is in Fanny's eyes. For Fanny it is an indication of a deeper moral malaise, and for her it is a deal-breaker.

Mary makes another mistaken assumption Fanny earlier in the same chapter:

“Oh! that I could transport you for a short time into our circle in town, that you might understand how your power over Henry is thought of there! Oh! the envyings and heartburnings of dozens and dozens; the wonder, the incredulity that will be felt at hearing what you have done! . ....... You should come to London to know how to estimate your conquest. If you were to see how he is courted, and how I am courted for his sake!

Of course Fanny would gain no satisfaction at all from provoking such reactions among the London crowd, and indeed would dislike that sort of attention very much. That Mary assumes otherwise shows how little she understands her.