Sorry, but Apple's policy clearly states that, "all sales are final" > iTUNES STORE - TERMS AND CONDITIONS
Since it's your iPhone, you are responsible for billing.
This may come in handy in the future > iOS: Understanding Restrictions (Parental Controls)
It is impossibe to make purchases in an iTunes account without supplying the password of the account holder. Providing her with the password that allowed her to make purchases would be consent.
Normally the password is good for 15 minutes and a number of purchases could be completed in that time. If you wish, you can activate Restrictions on your iPhone and one of the restrictions is to require the password immediately with each individual purchases.