As the above poster said, there is little to no chance of her being pregnant, but the best thing to do is to test in a few weeks time (if she hasn't had a period by then) and check. There's no point checking sooner as it's hard to detect if you're pregnant.
The best times to do a pregnancy test are either around three weeks from the date of possible conception, or around 4-5 days after a period is due/any time after that. That's when the hormone levels will be high enough.
Speaking from experience, I have used the morning-after pill before, and was not pregnant.
Anyway, if you have any doubts, get her to test (more than once to be sure!) and in the event that anything does come up, there is plenty of time to get that sorted out. In the UK there are plenty of free options. Make sure you talk to her about it, it's as much her decision as it is yours if it comes down to it.
But in all honesty I really think there will be no need to worry.
If it makes you feel any better, I had a friend who (very irresponsibly) relied on the emergency pill for contraception several times and has not been caught out. Once, she had completely unprotected sex and took it over two days later but still did not get pregnant. Hope that quells your fears!