If she goes to find her father now and found him, she would have to leave immediately to avoid trouble.
But before that, she must resolve Mu Si's matter. If Mu Si would be willing to go with her, with Heartbreak Valley, he would be able to make a name for himself. If he's unwilling to leave, she would ask Si Yue to give him a worry-free life.
Therefore, she would need to tell him the truth and let him make a decision.
"Mu Si, I know you are sad, but I must tell you the truth now." She patted Mu Si on the shoulder. "I know my father's whereabouts, and I will set out to look for him. After I find him, I will leave straight away and never come back. So, do you want to go with me or stay?"
She was leaving? Mu Si looked at her blankly.
His father was gone. Now she was leaving too?