As far as I'm concerned, at $600, I think BNB is the best. That's because the potential price of BNB could also become very high in the near future. However, since you already own ETH and BNB, I think popular altcoins like SOL, ADA, and MATIC could be your choice. actually XRP is also quite good, but it seems like it will take a long period of time before the price rises.
XRP was once in a very good position and was the best investment but now XRP is not in a good position. While all the coins have improved their prices a lot during the current pumping season, XRP has not improved its price much. The only reason why its price has not improved is due to the negligence of the developers of the XRP coin. If developers could responsibly move their coins forward, maybe XRP would regain popularity and be trusted by people.
I will ask him to go with BNB, Ethereum, Solana coins with his 600 dollars. If he can move forward with all these coins I think he will profit well in the next bull season.