User blog comment:Miskos3/Fairy Tail Chapter 488: The Two of Us, Always, Review/@comment-24442815-20160606194639/@comment-7418318-20160606221637

Mest doesn't need to visit the cell by walking there, he can teleport. He can just wait till Brandish is unguarded (which she was when Marin went in), excuse himself to go to the toilet. Then he just has to teleport into her cell, murder her and port back into the toilet.

I doubt anyone would even suspect him, much less be able to proof it was him.