It's not likely that your g/f is 25 days away from her period, although teenage women tend to have irregular periods and can skip periods.
Using a 28-day cycle. On day the day a woman menstruates, day #1, to day #12 a woman's body prepares itself to ovulate. On days #13-14 when a woman ovulates, the woman is most likely to get pregnant, but keep in mind that sperm can live up to 7 days in the female reproductive organ! On days #15-20, if the egg has been fertilized, the egg will implant itself in the uteral lining. If the egg has not been fertilized, on days #21-28, the lining will shed and the woman will menstruate.
So you'll have to start counting from the day your g/f started her period which is day #1. Count to the day you had sex and the condom broke. If that day is on or between #7-14, chances are your g/f could be pregnant. If not, chances are she isn't.
Best wishes and please don't buy cheap condoms, make sure the condom is the right fit, even if your g/f gets on the pill always use a condom. Also, try to convince your g/f to go on the pill/patch/diaphragm/implant or any other birth control method. She can get discounted rates if she goes to planned parenthood or a government family planning center.