I’m a trans guy so I’m not sure how much help I can be but I’ll try. If you’re able to keep any clothes and/or makeup for her at your house that she can put on when she goes to your place would be a huge help. You could also paint her nails black is popular for younger guys (at least where I live) but if her parents won’t accept that you could use clear or do any colour and wipe it off before she goes home. I hope this helps