Have him check the load side of the main breaker. Behind the buss bars.
There are two stakons there for N1 & N2 and if one has fallen off the generator will runaway.
I check stakons on every install as I found some are very loose and will fall off.
^^^ that too.:D. Although that would just make it want to run all the time. I read the OP as, "all is normal until utility power is lost, then restored"