After his dog bit him, should he pay for his medical bills?

“I had some friends in my backyard and they were playing with my dog. One of them began wrestling her, so she bit him hard enough in the leg that he needed stitches.
Now he’s demanding that I pay for his medical bills because I didn’t warn him that she would bite, but she’s never bitten anyone before.
I’m worried that if I don’t pay him, he’s going to try to get her taken away.”