It depends where you are playing from. On kongregate you can buy your friend a gamestop card. They can buy kreds with it and then they can use it them in game to buy anything Tank coins. Not sure how other sites work.
You can simply login to his account (you need his password) and buy it for him. But if you meant like sending him the tank coins you already own, not possible.
I would use a gift card and send him the details. Then he can input the details to buy coins himself.
@RumzMumzMario. His idea was prob the easiest an quickest.. if your friend is willing to do so. Or .. you could always give your friend the money for a month of deluxe... and he/she can purchase it themselves (There have been others who have wanted to incorporate a "gift" tank coin feature before..maybe if we get enough requests in the developer will implement it.) Welcome to the game / forums by the way