Well, at 19 she's not in danger of losing her teeth immediately (though I'm suspecting the one she broke and had root-canalled has either died or developed a cyst---been there too!).
But as Vinnie pointed out, the problem with these bacteria is that they don't stay in your mouth. They can cause all kinds of other problems down the road. And if the doc hasn't done a culture to pinpoint the specific bacteria, they don't know what they're fighting, and the antibiotics may only SEEM to be working.
Bring all this up with your dentist, if you don't want to go the perio route just yet.