Should I leave my boyfriend?

My boyfriend has a child he gets every other weekend. He goes to see her twice a week or more. Every time she leaves, he gets into these funks like when his mom died. When she's here, she's disrespectful, and his answer is “Well, I don't know what to tell you.” She swears and talks back, and he laughs. He and her mom have been apart for three years, and he blames her behavior on that and her being so young. We never have adult time when she's not here. She's the "golden child," and he won't listen to anyone.