If you firmly believe this was Comcasts mistake, open a police report. the truth will come out.
However, now that we live outside the US, there are a lot of uncertainty that could go horribly the wrong way. As I mentioned, my friend currently live in a rural area, it is normal to not be able to contact him for months. If the police could not reach him, what would happen? If the communication was cut during the investigation, what would happen? etc.
So you think I am asking too much from Comcast to provide one single piece of evidence that the account was opened under my name, before officially accusing my friend of crime (and create a huge burden on him)?