Singing is, among other things, a product of training. Obviously, that are limits to where you can go with your voice, but maybe you can train your singing voice to a more feminine one. I would sai to you prevent to sing in front of her, but it seems that is a really big matter to you. So, u can go training. But I really don't know if and how much it would change.
Being honest with you, seems more a problem that your go should deal that something you should change. Try being patient with her and help her. I'm sure you two can find a middle way.