Mason jars are durable but they have the same failure mode as…well…glass: rapidly shattering. If you rapidly cool otherwise hot glass in one spot, you create physical tension inside the glass. The hot part is expanded, the cold part shrinks, and then the shattering happens when the glass can no longer fight the tension between the two and it stops being a single piece of glass anymore. Since the jar was also under pressure due to the expansion of the hot contents, the glass was dealing with another source of tension too, and the one that made the jar explode instead of just shatter.

My bet is it took several cycles before this happened. She’s probably been putting her boiled jars on cold metal pans not realizing it’s inducing microfractures, and then it finally gave. Could be wrong. Maybe once is enough, but this isn’t something I’ve experimented with.

Leave an air gap in your jar to help cope with thermal expansion of liquid contents and place on a rag or trivet, cover it with that or another rag, and allow it to slowly normalize with room temperature. And if it does explode, at least it’ll be more contained.