I too would suggest consulting a good immigration attorney
Technically speaking, GC is for future employment, so if the company is not willing to employ him later (which I assume is the case since the person was laid off), the current petition would no longer be valid. You could still try, but it would be misrepresentation and therefore obviously risky. The only way out on this is if your friend can convince the company to atleast keep the job offer open, even if it is just for a day.
Employment letter is valid for 6 months although anything more than a month could raise eyebrows. Paystubs are subjective and are used for different purposes, to verify salary/ability to support, verify current employment, etc based on what the consulate feels is the need. Some consulates don\'t ask for it.
Another option would be to find another job and have the application transfered. I am not sure about what are the conditions for this, but at a minimum it has to be the same job description.
All the best