It is likely that whatever custom mod that was done by this kid of one of his friends, is the reason it exploded. Most likely whatever they did shorted out the lithium battery and it exploded.
Many lithium cells for critical applications have safety measures built in to prevent a runaway heat excursion. Aftermarket cells or heating coils substituted to gain more amperage in E-Cig are cited for such accidents.
http://www.cigbuyer.com/e-cigarette-fires-explosions-extremely-rare/
“Most likely whatever they did shorted out the lithium battery and it exploded.”
Agree, shorting or making the battery discharge faster would do it.