😞 read please

My 4 month old nephew spends every weekend with me; overnight and sometimes throughout the week. And when I have to bring him back he gets in such an unbearable mood that his mother can't get him to calm down. She has called me to come calm him bc he was crying so hard. Is it bad for him to grow such an attachment to me then not have me every night? I'm a preschool teacher so I know seperation anxiety can trigger this but I just don't want to hurt his development. Personally, I wouldn't let my baby spend the night with anyone at this age but I don't think his mother is mature to care about that. I was just wondering if anyone had any advice or personal experience with this and how do they or did they handle it? I get him Thursday evenings, keep him Thursday night, Friday, Saturday, and bring him back on Sunday evening orΒ 
sometimes Monday mornings.Β