She's lucky to have you as a friend: how great that she has that support from you. Sexual assault -- especially kinds like this, which so often are presented as something other than what it is, which is sexual assault -- is so isolating.
The thing is, Plan B isn't 100% effective: I wish it was, including so you could tell her it was. But Plan B + withdrawal (which is effectively what was used here since he didn't ejaculate) does really minimize her risks to the point where it would be incredibly unlikely. Mind, pregnancy also tends to take about 10 or so days to get to the point where it could make a period late in the first place, so if it hasn't been that long, her period is likely late for other reasons, like the stress of all this or the Plan B. But the only way for her to be sure she isn't pregnant is to take a pregnancy test: she certainly could take one now because if her period *was* late because of pregnancy, her test would also be positive. Know what I mean?
Now is probably not the time yet to mention this with her, but she will also have STI risks to contend with here: they're just as likely as a pregnancy would be from unprotected sex. Maybe in a month or so, if she hasn't brought it up herself, you can make a date for your own testing and invite her to join you or something?
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead