glad you are ok. something grinding through the caliper body long enough to put a hole in it without you noticing until failure sounds pretty unlikely though. if there is that much friction you would feel it and likely smell it. plenty of cars run with minimum clearance between caliper and wheel. just need to get a sheet of paper clearance. still in racing shit happens. proof in pictures would go a long way. otherwise sounds like maybe you were going too fast for your experience and/or condition of the car. doesn't help that you picked up a friend right before.
"It gave you amazing satisfaction, but anyone who says he loved it is either a liar or he wasn't going fast enough." - Jackie Stewart on racing at the Nurburgring