Call me the skeptic, someone needs to be one.
I checked the plug and sure enough, it was grounded, how do you suppose he did that?
How did you check the outlet because I am not sure.
Below are a few images of standard 120 VAC outlets.
Was the outlet without a ground the way it always was? Meaning was it just a 2 prong outlet sans a ground lug? Old style outlets like this did not have a ground and frequently in older homes using knob and tube wiring there was no third wire ground. The boxes that housed these outlets were not generally grounded as it was a two wire system. If this was the case I would be curious how the outlet was grounded?
A common trick used is to install a new three conductor outlet and tie the ground to neutral, as neutral does indeed tie to ground at the circuit breaker panel (or fuse box). However, this practice is strictly against the NEC rules. Read into that, it is illegal.
If you look at the 120 VAC outlet images you will see that on a standard outlet if it is placed in a grounded metallic box, ground will in fact be made without connecting to the ground screw. Outlets where the ground lug is not connected to the outlet tabs are an exception and are color coded and marked to identify them from standard outlets.
I can remove and replace an old 2 prong outlet with a new 3 prong grounded outlet in 5 min. However, without the presence of a ground, I can't ground it legally in 5 min. I can tie the ground on the outlet to the neutral and testing with one of those off the shelf testers will show it to be good as gold. Won't be legal but it will look good.
Thus I am a skeptic....
Ron