There's definitely something weird about who the cat chooses, most people over at reddit have noted that no matter what, if you have the red prince in your party, the cat always seems to chose him. Regardless of skills.

I confirmed this myself when I manually walked each party member over to the cat individually, however the cat only triggered when red prince came, who was last to come.

I don't know what the selection process looks like without red prince though.

It clearly says it in the first sentence of article above. Read first.
"Black Cat [...] follows the first character in party it sees."

The person our cat followed in our 4-player coop was the ONLY person without Pet Pal. It was pretty funny that it followed and gave the skill to the only person that couldn't talk to it.

It's a little buggy sometimes. When you walked in, did one of the soldiers say "What is that thing?" That seems to be related to triggering the dialog.

Tried what you said and even when the soldier noticed that cat and said, "What is that thing?" nothing happened, nothing triggered except the same old dialogue with the judge. =(

That whole thing does not work. Looks like someone is trolling hard.