For sure he knows that, if his money is diversified then it's reasonable that there are allocated money in some CEX, the only problem here is the CEX, FTX is a good exchange back then, yes but at that time there are many good CEX exists that qualify the most in terms of accountability. Also, it's for the convenience especially if you want quick trades and always do P2P transactions in your country, and some of the CEX are insured, it's like storing money in the bank already. I get it, it's more secured for a non-custodial wallet but can't deny the fact there's accountability to CEX too that makes you feel secure to put some money in. The current era of digital is being developed, a lot of people storing money to digital banks nowadays and people find it secure as long as the bank have a good reputation, same with exchanges. Also, maybe he's trading and using the platform for the leverage or for staking, no one knows better ask him than questioning his knowledge.