What to do and say
It is best to call the customer, it's frightening and they could be burned or injured.This is a known issue but it is extremely rare. It has happened 2-3 times in 15 years and millions of glass cups sold.
Our glasses are tempered to make them heatproof and robust, the same process as pyrex. Exploding glass is a known issue but it is extremely rare. It is thermal shock, it can happen if you pour cold water over a hot cup, or if you drop a hot cup on a cold hard surface. The sudden change in temperature causes the glass to expand or contract unevenly and break and most likely, there is a flaw or imperfection in the glass that triggers the break.
It can be frightening and we are really glad you have not been injured. Please accept our apologies and a gift voucher to the value of $100/50 GBP.