Buying for someone else is not a problem, but when he decides to sell, he might have some problems, especially if profit from cryptocurrencies is taxed in your country. He will need to prove that he bought those bitcoins.
buy for someone else we do with a buy and sell system, we can take advantage of services what we do, or commonly called getting commissions from buying and selling, because the value of bitcoin changes very quickly if you buy for someone else and in the long run is very risky, because of the purchase price figures will always change from time to time always happens.