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.
I don't think it will cause any problem at all especially if you trust that someone, but if we are in doubt then just buy bitcoin to legit exchange so you won't have any trouble. So, it is better to buy at legit exchange or those companies who does buy and sell in bitcoin rather than in somebody.