Re: "not part of his defence strategy"?
If he did do it deliberately, he'd have to have shorted the cell out or severely abused it, as batteries - as a rule - don't just pop. They get shorted, overheated, or otherwise abused and then fail, typically through thermal runaway and venting.
There are exceptions (poor manufacturing practises, as seen by Samsung recently) but these generally don't affect the sort of cells used in e-cigs, which are normally cylindrical and are a mature, stable tech - not bleeding edge, trying to be as thin as they can flat cells.
If he had the cell in his pocket with a length of chain, his keys, or similar, then all he'll have proved is that his client may have done the same, rather scuppering his case.
It does all smell a bit fishy, frankly.