(ONLY WORKS ASSUMING THAT FRIEND #1 AND #2 DO NOT KNOW EACH OTHER.) I would tell friend #2 that friend #1 is a HUGE fan of Shakira and she just broke up with her boyfriend and is having a really hard time recently......blah, blah, blah.......and would you mind if I took her instead because it would REALLY brighten her up (she has been a wreck, nothing I do can make her feel better...all she does is binge on Ben and Jerry's and play depressing music.....)? #2 should understand if she is a friend. I would take #2 to lunch, dinner, other concert soon though, so she doesn't feel slighted.
I am sorry you are in this situation.......it puts a damper on a great thing- FREE TICKETS
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Normally, honesty is the best policy (and ALL my roommates say: be honest!......but this is honestly what I would do.......).