Provided he is backfeeding a 220v receptacle, connecting the two hots on each line and the neutral to the neutral, then there should be no problems. I use a ground which serves to ground my generator frame through the house ground. It sounds like your friend has one of his phases connected to the neutral of the backfed receptacle. Remember that 220 is two 110 lines 180 degrees out of phase. The neutral exists to create a differential of 110v on either leg. Measured across the phases you will see 220v but either phase to ground will only show 110v. Check your wiring and I am sure you will find the issue. Oh and be sure to disconnect from incoming power as others have noted because while getting your generatr blown by the power coming on would be bad, it would be worse to injure a utility worker that wasn't using his personal grounds.