Hi jnjn,
As you have noticed, the Scarleteen staff has not responded to your previous posts because we do not respond to pregnancy scare posts. But we do answer questions relating to bodies, gender, relationships, sex & sexuality, and sexual health, among other topics. Since your menstruation question fits into these categories, and is not directly about a pregnancy scare, we will happily answer it.
The answer to your question, in short, is: there is no way to know for sure. Cycles often do not run like clockwork, as your girlfriend's experience has shown.
In Scarleteen's main article on menstruation, Heather explains: "You might even find you skip a period sometimes, or have a way shorter or longer cycle seemingly out of nowhere. Periods and fertility cycles can vary a lot or be very different sometimes because the reproductive system is pretty sensitive: all the various changes in our lives -- rest, activity, how and what we eat, stress, health changes, how our bodies can change after pregnancies, even a move from one place to the next -- can impact cycles and periods." The article also says, "So, if your period is not exactly the same for months or years on end, no need to worry: that's more common, not less. A regular period just means your cycles are around the same number of days each time, with room to give or take a few days earlier or later. (A late period, for the record, is generally considered so only once it is five days later than the latest it is expected.)" You can find that full article here:
On the Rag: A Guide to Menstruation
You may also want to check out this piece:
M.I.A or, Dude, Where's My Period? It explains a bit more about the different reasons why a period may come at different times, or not at all.