...Muriel probably could see The Confession through the window. I think it was already established in the fandom.
But also - they were with Metatron at the moment, who had to explain them their new appointment (I wonder, in what words?) while also watching the drama unfold through the same window.
I wonder if Muriel can say something...interesting about it. I don't know what exactly - the very fact he was watching (though if it would be told to Crowley in S3, he probably already deduced it?)? Some of his expressions or words - could he be a bit careless with them while being around "a dim one"?
Was there a reason Muriel was a bit sad in the bg when Metatron entered the shop after Crowley leaving it? Can they provide some unexpected (even to viewers) view of what they saw through the window?
....or maybe they just spent all their time reading and were sad because Metatron said to them it's time to start their new responsibilities and put the book aside. That's possible too.