Marissa finds a Comic book store owner who testifies in court as an "expert witness" on graphic novels to comment on Jay's artwork. In reality, an expert witness must be certified by the court before they are allowed to testify. The expert must produce evidence of their education, training, publications and experience in the subject area. You can't just testify using a fake British accent.
The show is infamously not shot in Chicago but the building depicted as the Dirksen Federal Courthouse is in no way similar to the actual Mies van der Rohe black glass and steel federal building, a Chicago architectural landmark.