His battery is also new another thought could it be the O-rings on the regulator.
This is exactly how mine went, From running perfectly all day to not starting when I came back to it from the shop. After a little investigation It would start with some Easy start and would run fine all day. Occasionally it would start and run OK. I then read on here about the HP regulator seals and sure enough one was split.. So if it starts with a squirt of easy start then as Steve has suggested it could be the HP regulator seals