my mom was totally against piercings. i got my first at 18 after i met my eventually to be husband. she had a fit! (my navel for heaven's sake).
then i got my tongue done next. i thought she wouldn't know, but she did know. my tongue was swollen for about a week and what did my mom make for dinner that night? spaghetti! it was so hard to eat. the stupid noodles kept sliding around the ring and i had to chew ooo so carefully.
next night? roast. the kind that falls apart in your mouth. the little "strings" of beef did the same as the spaghetti.
then she had had her revenge and didn't say much else. (i was working, going to college, and legal, so she didn't give me too much hassle).
i had the tongue ring until maybe i was 28. i used listerine religiously to keep it clean. i had a shorter barbell with smaller balls on the end. (i had a cute playboy bunny one, but the stuff in your mouth collects under the plastic. stainless is best.) also, try to get a SOLID barbell where the balls screw ON not one where the balls screw IN. (same as above reason)
also, she may have gotten her tongue done because it is easier to hide. it's rebellious, but still secretive. she could have gotten a labret or septum!!
maybe you could have her exchange the tongue for the nostril and get a diamond
i'm just wondering where she got it done.
another thing is that the tongue is supposed to be done at an angle and not straight up and down. she can brush under her tongue and i recommend that. i never chipped any teeth (but i didn't have the "clicking" habit that some people think is cool/sexy. --oooo look at me, hear my metal--). she should try not to chew on it!!
also: she might come to you and tell you first before you let her know that you know.
i know it's funny, but since i've been pierced and have tattoos i feel a bit more lenient.
sorry if i wasn't much help. i do have still and have had other piercings done. the eyebrow hole is still visible, but normally only after i've waxed.