If you really care about your friend better to tell him everything, and teach them how to play in the market. Im not saying that you idea is illegal, its legal as long as you are committed but again teaching him about cryptocurrency is the best thing that you can do, let him enjoy the cryptomarket.
giving an understanding of crypto is more useful than we buy for others, because an understanding and knowledge is more useful in the future, with a good understanding can make someone able to buy and sell independently.
what if his friend already knows how to use cryptos but he is busy to take care of his investments that why he ask his friends to pilot his account .
if thats the case then theres no need to teach them about the basics of understanding the cryptos instead we will only buy for them because we are his friend . that happend to me once because my friend ask me if i can lend him some cash to buy btc .
what i do is i buy him a btc directly using his own wallet and voila , he have now a btc on his balance .
just dont buy btc if you dont know the person because they might scam you .