As BooksandCorsets said, straws in the mouth work with limited alteration to the lips. I also can hardly breath through my nose (it's been broken too often and the air ways are messed up) but I've had 4 face and/or head casts done already. I put a straw in my mouth and it is usually fine. The last face cast I did, we did put straws in my nose and I managed to get through the process without suffocating. However my friend kept making jokes so I have a slight smirk on my face cast now. The best thing to do is leave the mouth area till the very last so that she can breath normally for the main duration of the cast. That way she only has to breath through a straw for the last little bit. As others have suggested, if she has never done a face cast before, practice with damp towels first to get the feel of it. Also have a safety "signal" she can give with her hands if she needs the mask off ASAP.