If Mother Lu could be manipulated by anyone, she wouldn't be Lin Fen.
Her biological son, Lu Chen, is now morally corrupted,, and she doesn't show him the slightest leniency, let alone a Li Da'an.
"If you want to go, then go, we will accompany you to the end!" Although Mother Lu had retired, she had not given up on learning, frequently engaging with advanced technology and the internet. She knew all too well about those online schemes—otherwise, she wouldn't have warned her daughter about some parents who find their adopted children and demand to be cared for or ask for liver, kidney, or bone marrow donations, which were all things she had seen on the internet and television.
She believed in her daughter, and also believed in Tang Tang, whom she'd watched grow up, and was confident that neither would succumb to Li Da'an's threats.
Even if Li Da'an really did so, they had ways and reasons to deal with him.