So, finally sorted out… the font in his file didn’t embed because it was a ‘free’ font that didn’t give permission. He used the font in a layered photoshop file that served as the background of an Indesign ad. The ps file flattened when he generated the pdf, so that wasn’t the problem. However, he had tried to use the font in the ID file. He thought he had removed it from ID, but it was still being used as a spacer in one place. So the problem was an unpermitted font being used in Indesign, and being invisible.
He said saving it as raster at 9pt wasn’t ideal, but he didn’t think the client would spring $30 for the legit font, and he didn’t want to eat it, so he went with the ‘free’ one.