The actual problems here...
...well, actually, there are a few.
First of all, he took a battery fresh from the charger, and chucked it into a low-resistance mech - that's not a great idea as the battery is a bit chemically volatile following a fresh charge. This is fine in most cases, but in high current devices, it can be dangerous. This is the nature of lithium based batteries in near-capacity-draw situations.
Secondly, his mech was a hybrid - where the positioning of the posts at the bottom of the atomiser are critical to prevent a thermal runaway from a dead short. Hybrids are a fucking stupid design, because it's stupidly easy to get a dead short. You can probably work out how from this image.
Third - and this is the second worst kicker - up until recently, his shop had been maintaining his mod, including building his coils to a safe resistance, checking the clearance of the RDA pin, etc.
They had recently stopped doing this because the upcoming US FDA regulations say that if they do that, they become a tobacco products manufacturer, which would either bankrupt them, get them shut down, or both. So to avoid this risk, they stopped servicing e-cigs, just selling them - leaving the customer to DIY it. The FDA has only recently clarified that yes, shops can change pre-made atomisers, but the wording of the rule means that it's technically impossible for a shop to work on a mech like this due to it using handmade coils, and not be in breach of the letter of the law.
And the joint biggest problems here - by his own admission (later in in the postings on his FB page), he wasn't entirely sure what he was doing when it came to the maintenance of the device, because he'd left everything for the shop to sort out; he thought he was doing everything correctly, but sadly wasn't.
The other side of that, is that when customer comes in asking for a hybrid mech mod, the shop didn't ask "Why?".
Like kit cars, sporting rifle shooting and motorbike track days, hybrid mods are something that are always - without exception - best left to those who can seek them out, and demonstrate that they understand the risks.
Sadly, some shops really don't seem to give a fuck.
Which pisses me off, because if you look through that FB post (where all of the above info comes from) you'll see dozens of people inferring that they'll go back to ciggies, and sharing it saying they're going back to smoking because it's 'safer'.
So a slow handclap to the shop for failing to do any due diligence on their sales (if you're having to rebuild someone's mech for them because they can't do it, then they shouldn't have the mech, and that goes doubly so for a hybrid - end of story) and a slightly more sympathetic one for the unfortunate user for not doing the research on a device that in normal use basically runs as a near dead short on a cell that has about as much kick as a 4/10 shotgun shell if you get it wrong and pop it.
There's a good argument that mechs are, these days, irrelevant even in the low-ohmage world, just as they became a few years ago for e-cigs that had a resistance of >1ohm - yes, you used to get mech mods for 1.8ohm devices, before pocketable 20w regulated mods made them look pointless.
These day it's quite easy to get a three or four cell mod that will (just like in the >1 ohm world a few years ago) make any mech look like a complete waste of time, while also being effectively unbustable in normal use - short circuit protection, undercurrent protection, etc - while also being able to kick out 300w, more than most mechs can safely provide, regardless of what anyone tells you about their 'pulse' current.
I fully expect that we'll see pictures of this chap plastered all over the news for a while (because the rarity of such events is still newsworthy), all while every day three house fires are started by normal cigarettes.
In London alone....
PS: If you're interested in seeing what informed people other than me have to say about this, I'd recommend this video from VapourTrails.tv on Monday, where this was covered in detail, by people extremely familiar with the tech and the risks involved.