He found her washed up in a rock pool, green… blood, he assumed, matting her hair, as well as another cut on her scales.
Assuming he was dreaming, or dying, or just anything not real, he decided to play along, and pulled out his first aid kit and attended to the… Mermaid, he assumed.
Eventually she woke up, and patted her head and… stomach, he assumed, looking confused at the bandages on them.
She began to speak rapidly in a language clearly not human, and reached out to him for… help back into the sea, he assumed.
He assumed wrong.