This article is from the FAQ, by with numerous contributions by others.
The labor certification is valid, but your friend may not get a GC. Reason:
Labor Certification is for a job in future. But for the GC application,
that job must be certain to be available right now. Currently, that is not
the case. If 6 months (or more time) later, your friend gets rehired by
the old company, he can go ahead and apply for GC as long as the job he
gets is the same as the job for which Labor Cert. was obtained.