Lady Susan doesn’t say whether or not Frederica was also planning to elope after she ran away, which would suggest a lot more cunning on Frederica’s part than Lady Susan gives her credit for. In fact, although Lady Susan is sarcastic about it, she seems almost impressed by Frederica’s daring—it certainly doesn’t align with the shy, meek girl Catherine described to Lady De Courcy. In fact, it implies that Frederica might really be more interesting than she appears when directly compared to her mother, which would explain why Lady Susan is concerned that she’s come to stay at Churchill. This passage also demonstrates that Lady Susan hasn’t been entirely honest with Alicia: Lady Susan claimed that she didn’t want to marry Reginald, and that she wanted a good match for Frederica. If both of these things were true, she should have no issue with Frederica’s crush on Reginald (he’s just as wealthy as Sir James, after all). Either Lady Susan secretly
does plan to marry Reginald herself, or she just wants to control the situation and is angry that Frederica is trying to take that away from her. Unfortunately, Reginald doesn’t seem to care that Frederica likes him—genuine love has no effect on Reginald, since it’s “artless.” Reginald apparently responds better to Lady Susan’s manipulation, which always demands something of him. Frederica, meanwhile, is sacrificing her mother’s good will just to pine for Reginald, which proves that her feelings are strong.