I'm doing the Making a Snowman writing challenge. How is a ball of snow a person that requires lui? (Girl was rolling a ball of snow to make a head)

Graham,

The "lui" represents the snowman, not the ball of snow:

Martine roulait une grosse boule pour lui faire une tête.

I agree that the text in isolation is ambiguous, but this has been clarified by a hint under the text:

Martine was rolling a big ball to make it a head.

HINT: "it" = le bonhomme de neige

[In English I think we would say "Martine was rolling a big ball to make him a head".]