He did a 1G roll at altitude. Nothing unsafe about it. In fact, the occupants on the plane wouldn't even feel it beyond the typical turning feeling if they weren't looking out a window.
Just to be pedantic, it's impossible to execute a roll and maintain 1 G throughout. Typically, something like 0.5 Gs are maintained at the top of the roll, but up to 2 or 3 Gs are seen during recovery of level flight. That would be noticed by any passengers.
Further, what Tex Johnston performed was a modified aileron roll, not a true aileron or barrel roll. He started the roll while in a climb, and ended in a shallow dive to keep the Gs limited while in the roll itself. Recovery from the dive would see in excess of 1G.