Being built on is not the same as running on
I built my computer on my bed (obviously also easy to get a static charge built up), although, again, I suppose if that's the first time he's done anything with building computers and didn't take precautions it would be pretty bad to build it on the carpet. I didn't have another place but the carpet to build mine and I always go way out of my way to not touch any pins or anything and again, usually touch anything I see that'd discharge the static so I guess it's mainly about how cautious you are, still not very smart to do regardless even if you know what you're doing.
I don't think it's as common as people make it seem honestly.
Edit: Holy stuff, this made me think of when I was building my computer, I just realized how often I discharge myself subconsciously when dealing with electronics, I'm not even sure what caused the habit considering I don't have THAT much experience to where I think it'd cause that.