If he is comfortable using Kucoin, I don't mind it, I understand that everyone has different tastes and criteria in choosing which exchanges to use.
Regarding your advice to choose coins that are profitable for the long term, there are quite a lot of them and usually have a high volume.
Like BTC, ETH, BNB, ADA, SOL, and DOT are potential and safe for long-term trading. From some of the coins I mentioned, now I focus on trading in BTC, ETH and BNB. Because if I trade too many coins it is difficult to monitor it.
Kucoin is one of the best in the security department as well. My boss back in the day (not my current boss) went to see kucoin for a meeting they had. It was about him getting the coin listed and wanted to have a face to face meeting. He says that they gave him a tour of the building as well and they have shown him how they secure coins as well, cold wallets and how they are managed, the servers, the security they have and all in all he was quite impressed about how everything looked solid.
This doesn't mean that they can't be hacked, but at least they made it as hard as they could, hackers could still find a way to do it of course. I agree with the long term ones, they are high liquidity and very strong coins that could have a big potential in the future, it may take a while before we could see it increase to huge numbers again but they are at least trustworthy enough to put money in.