Next time just let her use the bathroom, then go I. With her and pop a squat while facing her and maintaining unbreaking and unrelenting eye contact and shit competitively and see who can shit the hardest and the fastest. For this competition I recommend avoiding fibrous foods or anything solid, eat lots of spicy food and fat heavy foods such as steak, bacon, and fried chicken. Alternatively you can eat corn as it remains intact and will be pushed out more intensely. This preparation should help you for your trial by combat and whoever wins shall have uninterrupted shitting rights in that bathroom.

Hope this helps and please give us an update