Well, I wouldn't have someone's Kindle on my account that I couldn't trust...my brother's Kindle is on my account so that we can share books. I trust him not to start one-clicking madly on the Kindle. He pays me when he does buy a book.
There are parental controls, but I don't think putting those on her Kindle would endear you to her.

I do think there is a way to require a pass code to actually buy stuff on the Fire, I think. Off to look, unless someone here beats me to it.
Edit: You can use the Parental Controls to only rquire a password to purchase content from the Amazon Store and the Amazon Shop App. You could add only that part of the Parental Controls. Go to Settings (swipe down from top), then More > Parental Controls.
I think you will see Parental Controls on/off. Turn them On and I think you will have to enter the password you want to use. Make sure it's something you can remember, because the only way to get around it is to reset to factory defaults. (I'm not sure about the previous because I had already set a password for Parental Controls.)
Once you see the list of Parental Controls, look for Password Protect Purchases. Set that to On.
But again, I'm not sure this would endear you to her. It might be better for her to have the device on her own account.
Betsy