[Nagoyankee]

People write their pet names however they like to. They use all three writing systems. Which one to use largely, if not entirely, depends on what name it is. Many pets are given "Western names" such as John and Suzan, in which case they would surely be written in katakana.

As KyleGoetz says, 岩井 is a family name. I wouldn't name my pet Jones or Hernandez but hey, it's your pet. 祝 does mean celebration. A Japanese person wouldnt name a pet 祝 but then again, your friend isn't Japanese and her dog has already been named as far as the sounds, correct?

So, feel free to write it anyway you want to. 祝、岩井、いわい、イワイ....