Sometimes airlines (especially their partners or on promotions) seem to short you miles that you're owed and sometimes you get extra for some unknown reason. Over time these seem to even out (at least for me) but with such a large mistake (100,000 miles is almost three coach tickets to Hawaii). I think your friend should let Delta know about this. They may give him some miles for his honesty (or maybe nothing), but at least he'll sleep better knowing he did the right thing. Trying to use the miles that he knows aren't his is wrong.