Definitions as circular a must?

First, your friend was simply incorrect about the definition of "humanist". There are several definitions or variations on the definition of "humanist," but simply being human is not one of them. That aside, if a humanist was simply a human, then it would not be circular unless you defined "human" as "humanist." If the two terms were the same, that needs to be made clear, as opposed to one term being part of the group of the other term. Here is where we get into the "not very helpful" category. Some examples:

What is a dog? A canine. What is a canine? A dog. (not at all helpful, fallacious).
What is a dog? An animal. (somewhat helpful, but not fallacious).
What is a dog? A domesticated carnivorous mammal that typically has a long snout, an acute sense of smell, and a barking, howling, or whining voice. It is widely kept as a pet or for work or field sports. (helpful, not fallacious)