I used to attend daily mass quite frequently at a Catholic university and often some of the priests would join daily mass as regular members of the congregation. I try not to notice the people around me, but once I noticed that one of these priests was reciting the words of consecration from where he was standing in the congregation. (I know we should have been kneeling, but that is an entirely different matter.) I didn’t think anything of it, and said to myself that he’s a priest and he can do it.
Could a priest who is not in any official capacity as celebrant or concelebrant pray these words? I think I remember seeing a mass from St. Peter’s in Rome where a number of bishops who not up on the altar were reciting the words along with the holy father. I’m not suggesting necessarily that the man referred to by the Original Poster was a priest, but if he was would this still be a problem? Just curious.