@Glennn,
Warn them... hopefully, it will never happen to me, but if it did, I would hope that some kind soul would alert me to it... although, if the tire is
very low, you can sometimes tell by the car's performance
A friend plays golf regularly, you don't... one day they ring and ask you to help make up a foursome because their original partner can't make it... there is $$$$$ involved! Do you go, and hope for the best, or beg off?