You shouldn't have to go to the bathroom to express. They have to privde you with a private space & time to express. So if you have been told its ok to ask this other women to leave then just do it. its not your fault if she turns up late. Also if your comfortable doing so, just express while she is there I bet she will probably just get up & leave on her own anyway. Or is there another room you can use that isn't going to bother anyone if your there.

I would talk to your boss. Let her know your feelings re the temp & see how it goes.