there really isn't a way to "limit" downloads, only block them. (which sounds like it might be a good idea) VZW customer service can block any additional DLs from happening.
As a side note; watch out for what she has already purchased, it can still be used. As an example; if she has VZTones Deluxe loaded on her phone, she can still purchase ringtones after the block has been applied. You may need to remove some existing apps from her phone