Can I give away my birthday gift without it being rude?

My (17m) parents gave me a $30 Starbucks card for my birthday. I go often but my little sisters best friend (14f) is obsessed but always broke. She mentioned she could spend the card up and parents would never know. I am tempted to let her but I know my parents would not be cool with it. Is it a good idea even if they never find out?