With that much LR
you can definitely add another fish. The only thing you have to consider (other than current inhabitants) is the adult
size of whatever fish you add. This is why mhaynes01
said a yellow tang wouldn't work, and you'll probably get the same response from most people here. They need plenty of swimming room and at the very least a 5' tank. It's not good practice to buy any fish that will eventually be too big for your tank, and your friend probably didn't get the correct advice when he started out.
As for which fish to get, the choice is really your preference. There's too many to choose from. I'm quite partial to dottybacks myself (fridmani pseudochromis).