Hi Ali
She's not making the association with the toilet being outdoors so your friend will have to start again with basics to try and break the habit. Indoors is her toilet in her eyes and you will have to change the way she thinks to teach her otherwise.
I would suggest that she gets a crate and pops her in there overnight. She may not want to soil her immediate sleeping area so that may be incentive to keep clean or at least the mess will be contained. As she is doing three overnight

I would initially get up through the night a couple of times to give her the opportunity of toileting. She's used to doing it then. Loads and loads of praise if she does!
Lara x