my 4 month old got bit by a dog

Saturday morning I left to work and left my son with a family friend because she had been asking to watch him so I dropped him off a couple houses away from mine. I called her during lunch and texted her on my breaks. After I got off work I called her telling her I was on my way, she told me he was asleep and would walk him over herself when he woke up. So about an house later she gets home and tells me there was an accident. She then continues to tell me that her dog (a golden retriever) got a little excited and thought MY SON was a doll and "scratched" him. Me thinking it was a small scratch didn't really mind it and told her it was ok that he would be fine and thankfully it wasn't anything worse. My son fell asleep after that and I didn't check his leg until the next morning and noticed how big it was!! You can see the bite marks!!!!!! My mother in law came in my room and told me she spoke to her friend and she told her the truth. That the dog actually BIT HIM!!! I'm furious my poor baby!!😭😭😭😭😭😭😭 while I'm at work thinking he is being well taken care of, I feel so irresponsible and heartbroken. Every time I see his poor little leg makes me want to cry💔💔💔💔