Plan b and pregnancy test?

Because she'd already ovulated when she took Plan B. It can't put the egg back in the ovary, and contrary to early reports, it doesn't do anything to prevent implantation. That isn't considered part of the "failure rate," that's just a basic fact.

Have her go to her doctor and get a blood test, the plan b could be interfering with the hormones that detect pregnancy.

She conceived earlier than you think she did. If you had sex earlier on in that week or month, that is when she got pregnant. For the pregnancy test to work, she would have had to have enough HCG (pregnancy hormone) in her body for it to register a positive result. Therefore, it was already too late by the time she took the Plan B, she had already had the fertilized egg implant in her womb days before that.