Why does Mrs. Marwood tell Lady Wishfort she has seen Foible with Mirabell?

Mrs. Marwood is scheming with Fainall to frustrate Mirabell's plans to marry Mrs. Millamant so she says she saw Foible with Mirabell. Marwood uses any method available to ruin Mirabell's plans after she overheard his true opinion of her when she was hiding in the closet