It wouldn't hurt to buy her something religious - at least, that is what the celebration is centred around and even though you might not be a practising Catholic, it does show that you have some sort of awareness of the faith!
Flowers might be a nice gift and as has already been mentioned, possibly an item of jewellery.
Even though many people make large celebrations out of Confirmations, in practice, you don't need to provide extravagant presents - if you are just a friend, it is generally accepted that a nice card and possibly a small gift is perfectly generous.