A 68PG/38VG blend should be thinner (less viscous) than a 50/50 mix. And, therefore, wick
better. Thinner mixes might cause leaking problems; but shouldn't increase chances of dry hits, or burned out coils. I don't think it is a PG/VG ratio issue.
Did you use a pod/coil that had already seen use? The pod could have been near the end of its normal lifespan and he is erroneously blaming the liquid.
If it was a new pod; did he allow for complete saturation of the new wick
before vaping? Premature vaping could burn out the wick and/or coil.
Could it me a simple matter of a crappy pod? Smok isn't exactly at the top of the heap for quality consistency.
If those aren't the issue, I am in to think along the same lines as
@Izan, and consider the flavor concentrates. TFA Watermelon Candy alone, may contain as much as
10% Maltol (reputed as a gunker). I don't recognize any of the
Apple Candy ingredients as gunkers, but that doesn't mean anything. CAP doesn't offer any sort of details on their concentrates, and I'm not sure which actual strawberry you used (they have a Ripe, Taffy, kiwi, Cream, and Sweet variation, but not just "Strawberry"). But, I would not be surprised if both the strawberry and v2 VC had their own sweeteners as well. Combine them all and use 18% to volume, and it could well be a gunkier type of recipe.
The bigger the coil, and the more wicking material available, the longer a setup can survive a gunky e-liquid. So, I could see a full-size atty fair better, than a pod with a tiny coil and minimal wicking material. Also, the fixed temperature/power output, may not be ideal for the recipe's composition.
Like Izan, I am just brainstorming, but these seem just as possible as an incompatible PG/VG ratio.