If Bo Junyan wanted to do something that his parents didn't agree with, so much so that his mother would fight with him about it, making his persuasion useless, he would use her greatest weakness to force her to agree to him.
He had used the same trick on his grandfather.
For example, the first time his grandfather had dealt with Mu Huan, he had directly given up his right to be his grandfather. However, later on, after Old Master Meng lost this right, he no longer had any weaknesses. On the contrary, now that he became old and fell sick, he couldn't be provoked at all. If anything happened to the old man, his mother wouldn't be able to take it. This became Bo Junyan's weakness instead.
Mu Huan knew that he meant what he said. That was why she was especially angry!
She was angry at him for using her best friends to threaten her!