I would think that in that situation, it would be best for her to spend the night with her friend when she is not having visitation with you. From what I have seen my friends go through when sharing custody of their children, they do not get near as much time with their children as their ex wives do. So, I would have a talk with her about how much you enjoy your time with her and that since it is limited, perhaps she could go to sleepovers with her friends when it wasn't your weekend to have her.