Most tanks these days that are designed for mouth-to-lung vaping are labeled as "MTL" tanks. They generally run a coil that is around or over 1 ohm, and you vape them with tighter airflow and lower wattage, say between 12 and 16 watts. I don't think there is a coil for the Guardian tank that is over 0.6 ohms.
I mix my own juice at 35% PG and 65% VG. A little more or less than this is barely noticeable. He just would not want to vape something as thin as 70% PG. Even 50% PG is a little thin for sub-ohm. And Max VG is probably too thick for his wattage level - Max VG is anywhere between 10 and 25% PG.
Next thing, important not to vape the tank dry. One good hit off a dry coil, and it's burnt, you have to toss it. That nasty burnt taste isn't going away.
As far as charging, what kind of battery is in his mod? It's important to have a good battery, and the higher the mAh rating, the longer it will last between charges, if it's a good quality battery and not some cheap Chinese mess that is nowhere near as good as it pretends to be. Sony, Samsung, LG, Vapcell, Golisi - all of these are good brands and if they are bought from reputable dealers, are the best you can find for vaping.