I'm not reading that Plan B causes a person to skip a period. In fact, the manufacturer seems to be saying the opposite: "The only way of knowing Plan B One-Step® has been effective is when you get your next period, which should come at the expected time, or within a week of the expected time. If your period is more than seven days late, you may be pregnant.”
Plan B One-Step® (levonorgestrel): Frequently Asked Questions
It is entirely possible that your friend was pregnant and miscarried early.
Do you have a source of info which is different from mine?