Not to mention he'll be spending a lot of gil repairing it. The larger the rank gap, the faster the wear, along with the need to spend on G5.

There is no standard scaling formula for equipment in FFXIV. Every equipment differs. The only constant is that the gear scaling formula will be such that you can upgrade to the next consecutive rank of equipment just after the mid-point of the optimal ranks.
E.g.
Upgrade Rank 7 to Rank 12 tool at Rank 10.
Upgrade Rank 12 to Rank 22 tool at Rank 18.

Therefore it is possible your friend is telling the truth, depending on the equipment he is using, just that it won't be very economical.