As far as her hymen tearing from using, by the time a girl gets her period, there is a hole in it already. Besides, those can be torn from doing things such as riding a bike or playing volleyball.
Hymen sizes are different for each woman. Mine is very small, it is barely there. I don't believe I have torn it, and I am still a virgin. To locate your hymen put your finger at the tip of your vagina, right at the top and drag it down until you get where you would put your tampon - stick it slightly in and you should feel a tiny fold of skin (unless you broke yours, or maybe you didn't ever have one) Some people need their hymen broken by a doctor because it covers their entire vagina, others only have and having a slightly large hymen can be painful. I think if you spread your legs out you might feel the skin contract and it might be a tad painful if yours is still intact.
I don't believe a tampon would break your hymen. Unless you used a significant amount of force inserting it, because the hymen is soft tissue and flexible.
The tampon would just push on it a bit, but once it is removed, the hymen will go right back in position.