No, it is a sign that the nicotine is burning the back of his throat. 100% PG often lets through too much of nicotine's irritation. PG lets through the most flavor, most nicotine TH, most nicotine irritation (nicotine is a skin irritant). VG dulls everything PG lets through but adds thicker visible vapor. 100% VG is also very thick and does not work well with many methods of vaping. I recommend blends. For stuffing cartos try 20% VG, for reservoir cartos or tanks try 20-30%, for plain atomizer try 20-50% VG. Most real juice vendors offer PG/VG blends.
Then if it still burns, he could consider dropping his nicotine level 1 step and just take a few extra drags if needed to make up the difference.
Once all of the cigarette chemicals are out of your body, the one that makes nicotine more addictive is gone. At that point I think the biggest goal feeling the TH of the drag (and nicotine IS TH), inhaling, exhaling, the flavor and aroma. The goal is for that to be strong enough to please and satisfy but not so strong that it is uncomfortable or causes irritation. If it is comfortable for him to drag, he will get the nicotine he needs.