my mum struggles with mental health but doesn’t understand mine
It does sound as if your mum does want the best for you and in a way I can understand what she's thinking. You do need rest and to her maybe talking with your friend gets in the way of it. As well she might be thinking you are disturbing your friend, by calling in the middle of the night.
If you have already tried to explain and failed then I guess the way to go might be for your mum to have a talk wiht your friend and your freind's parents to see if there can be a set of rules that do not actually stop all contact, but tries to put in boundaries to ensure everyone gets some rest.
What do you think ?