HHelen By asking for it you would make it worse, because she would become aware that you did it.
I understand where you're coming from, but I think she hasn't got so much to lose at this point. Unless she asks she'll be wearing a belt with built-in secondary shield 24/7.
HHelen Yes, it's a big disadvantage. She could wear her belt without a secondary shield at all. Since nobody can get pregnant from oral sex or masturbation.
So we come back around to the question, why 24/7 use, why a secondary shield? Is the aim here to prevent more than just pregnancy? Only @Hannah can say... and only after she's spoken to her mother. Good luck, Hannah!