Re: Girl friends period
First if she's been on the pill then I'd worry and get a home test kit immediately. Its a simple and painless test that could allay your fears.
Secondly -- it is possible that she can skip living in a house with all females. Many times hormore clocks get adjusted when all of them will have their periods within days of one another. If she's new to this arrangement, it could be that her body is adjusting to other's cycles. This is fairly common occurrence.
I hope this helps'
GET THE HOME TEST KITs and if it turns out that she's not pregnant consider yourselves lucky this go round and make sure you BOTH USE SOMETHING.