If you don’t manage to convince her to let him stay inside permanently, I would suggest stressing to her the serious risks of moving an indoor rabbit outside during the winter, especially if it is a solo rabbit without a friend to cuddle up to and keep warm. Rabbits might be outdoor animals by nature but when they are acclimatised to indoors it’s like a person popping a jumper on and trying to spend the night in a shed with the window open – you probably wouldn’t risk it. Perhaps you could politely explain it is very dangerous to do this in the winter months and suggest allowing your pet to stay indoors for at least the duration of the harsh winter before beginning to transition it outdoors next year when the temperatures warm up to a more tolerable point for the bunny.
Maybe suggest to house the bun whatever way your mom finds comfortable eg in a utility room or somewhere out of the way?
Good luck! 