Feeling sad and guilty

I had knee surgery Wednesday and a friend came by yesterday with a pizza to visit. We put her dog outside with my two. We ate, chatted, and 30 minutes later we went to sit outside. Found her pup in the pool. He's old and has poor hearing but can see, has been here before, and always avoided the water. She started crying. i grabbed a dog blanket and started yelling for my husband. He immediately got the pup out and tried compressions but it was too late. She's the best, most devoted dog mom i know and she's devastated. She feels like she failed him in his last moments. She keeps saying - Why didn't i leave him home? Why did i make him go outside? Why didn't i go check on him? - and feels like she deserves the guilt and pain because his last moments were likely scared and struggling. I'm so sad for her.