It's possible. Pre-cum can have sperm in it. If it's been less than 72 hours, she can take Plan B. Either way, she'll have to wait to see if she misses her period. If she does, she should take a test according to the package directions about *WHEN* and HOW to take it for it to be accurate (if she takes it too soon, she can get a false-negative).
Best of luck to your friend. Tell her to use condoms or go on the pill--or scares like this will keep happening.