Binance Tax Question

Hello, just had a question regarding transactions made on an account. Recently my friend has managed to verify their Binance account and has been asked if they could purchase/sell coins for a family memeber, as in they will receive USDT/USDC or any other coin, sell that then proceed to purchase other coins > send back to other party. Now here is the tricky part, how would the tax implications affect them? Would they have to declare everything as a gain or would it be even be considered a taxable event. As the volume of the trades were quite high.