I assume she had an online check in b/pass with a 2D barcode on it? Surely when these are scanned through the various checkpoints a warning would have been displayed? Gone are the days where getting through security just required the screener to check the pass manually.
It may not have been that huge a security risk, but it points out a rather gaping hole in the IT used to check boarding passes, both on the part of LGW security (and whichever airports use the same system), and on the part of FR's boarding system (if indeed they use any?).