Not exploding. Bulging. Batteries can do that. Especially over time and wear gets worse. Worth bearing in mind that's a 2012 13" MBP in the picture, or possibly even earlier. So unless they bought it brand-new from the Apple Store, it's at least a few years old. Maybe even 4. Maybe more if it's a 2011 or older. It's the nature of the beast with lithium. I'd bet my bottom dollar they had "Service Battery" for a fair few months before it 'exploded'.
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That happens if you don't service your battery when you should seen it a number of times. AFAIK, that's a security measure to contain the pressure buildup that would actually cause the battery to explode.