OK Lets look at the resolution and expected accuracy. This tells me that he is working on extreme detailed parts. AND the time to run tells me it is an expensive part.
I have never seen a machine cutting a finish cut at that resolution that IF you did a feed hold you would not mar, ding, scratch, gouge a finish cut.
Also With certain Material you cannot interupt a cut and have the bit rub the same spot it creates a hard spot which will never finish cut correctly
NOw as the outage events are random we have to assume the worst case senario, it is in a finish cut mode. Experiance tell us IF you plan on the worst case senario the rest will take care of itself(;-)
Try it and you will see what I mean.
The only way out would be to RUN continuously or wait FOR a point in the program that the bit is NOT cutting material. That could be hours into a long program. The only way to be absolutly sure is to STOP in a tool change senario. OR use built in hold routines throughout the program AND the programs may not HAVE either.
SO where do you stop?

IF you are just cutting cheap plywood on a router please disregard the above
The generator is just a replacement for a large battery bank same thing only different name. Still a stored energy source(;-) He mentioned a UPS so they need a battery and it is EASY to add more capacity, add more batteries. LONG outages yep he has the answer UPS and a backup generator
Now from past Power company Power Quality Tech experiance you need to run the generator for at least 30 minute AFTER the outage event occurs. AS they bring the system back on line there could be cascade failures that will trip the system back off again. So give the system plenty of time to stabilize before you transfer back to grid power.
As to the stop restart part that would very simple to do ONCE you have the machine stopped. We do that all the time in plasma to stop and check the consumables in long cuts. There is a routine to start restart on this forum somewhere that we use to do it.
That is just my experience in outage recovery technics, Your mileage may vary (;-)