Tell her

"You what!" yells Sarah as he tells her he couldn't find a centaur. She turns around showing her barely used horse pussy "Do you think you can please this? I need a big dick, not human I want horse cock!" her pussy has specks of moisture appear on it just from thinking about horse cock.

"W-w-what about the dildo I got you?" Tom asks her, "It looks big enough"

"Well it isn't," she yells back at him and storms off. Her vulva opens and closes as she walks away, she is clearly in heat. Tom examines his options try the coin on himself or go look for that doctor.