I'd suggest playing by the rules and do not give your number away if you cannot run. That averts the problem or the potential of a problem.

I would say it depends on if the money is very important to you and getting it back or not makes a big difference.

If money is an issue - try talking to whoever gave your number away and see if you can get reimbursed. Unfortunately, not the person who ran..., but that is a different story.

If money is not as much of an issue, than I'd say let it slide.

Best of luck to you anyway,

Just let it be, it's not going to do anything but make a mess if you confront her.  It's over

It goes beyond money these days.  Most races require photo ID to pick up a bib, so how did your "friend" manage to get yours?  Also, for some race organizations (Rock 'n' Roll, for instance) having someone else run with your bib can get you banned from future participation in their races.  Forget the money.  I would let the person who gave away your bib know, in no uncertain terms, not to do it again.  Sounds harsh, but it's your reputation that is at stake.

Len

Most running events specifically prohibit selling or giving a race I.D. to anyone else.  I think you do need to follow up because, unfortunately, the person participating should have been disqualified and both of you could be potentially barred from future events.  (What if the person using your number had been injured?...what a real and future liability nightmare for everyone involved).  It is a shame that someone you know gave away your bib, and if this person is a member of a professional running/race organization, then he/she fully knew better, so that is just totally unethical and ridiculous.  At this point, I guess cover your bases for the future and that may include filing a report/complaint to protect your own best interests. 

Wishing you great progress