Mrs. Rushworth had left her husband's house: Mr. Rushworth had been in great anger and distress to _him_ (Mr. Harding) for his advice; Mr. Harding feared there had been _at_ _least_ very flagrant indiscretion. 
Mrs. Rushworth had left her husband's house: Mr. Rushworth had been in great anger and distress to him (Mr. Harding) for his advice; Mr. Harding feared there had been at least very flagrant indiscretion. 