As the saying goes, it's difficult for even an honest official to settle domestic disputes. Yan Yu did not plan to meddle in Zhou Hongyu's family affairs because it was easy to end up thankless and overwhelmed—who knows what she really thinks of her mother?
If the girl's true thoughts were "I wish my mother would love me once more", then rashly separating her from her mother would certainly backfire. If Yan Yu were handling it, he would find a way to leverage her mother's weaknesses, threaten her to act the role of a "loving mother" in front of Zhou Hongyu, and then gradually guide Zhou Hongyu to disillusion and desensitize her from the fantasized maternal love.
But if Zhou Hongyu simply wanted to repay the "debt of birth", then finding a place to confine and support her mother, ensuring her a high quality of life in exchange for her absence from Zhou Hongyu's life, would naturally be a better approach.
Ultimately, it all depended on what Zhou Hongyu herself thought.