First of all, he is not entering Thailand on VOA - Visa On Arrival. He is using the visa-exempt scheme. Those are totally different. While visa-exempt entry is for 30 days for free and eligible for 30 days tourism extension, VOA is for 15 day stay by paying the 2000 baht official fee at the entry point and is NOT eligible for tourism extension.
The rule is, the limit of TWO visa-exempt entries by land per calendar year. No written rule for the limit on visa-exempt entry by air .
But he has a good chance to be questioned at passport control, better have proof of accommodation, funds of
*****
baht cash, and onward tickets when entering Thailand again, and be prepared with an explanation as a genuine tourist.