She's a mother!
"She looked up at him, her eyes as cold as the eyes of a lioness. This was Juana's first baby - this was nearly everything there was in Juana's world."
As the father of a two year old, I've learned one very valuable thing: when it comes to the safety and protection of our little guy, there is no consulting with his mother. I don't mind him jumping on the couch or off the second stair; his mother gets nervous when he walks across the living room.
Juana not only feels threatened for her only child's life at this point, she also knows how deadly scorpion stings can be. For her to stop and try to reason with a stubborn male about the necessity of a doctor, could be the difference between Kino living or dying. Frightened mother = quick decision.