He said that, contrary to that statement, the third was never "banned", that the idea it could have been is ridiculous. He notes that as soon as organum began to generate some linear independence, thirds appear harmonically in the manuscripts. He notes that it was not used in a way that implies it is a perfect consonance for a couple of centuries but that's a far sight short of it being "banned".
He said that he would look for a harmonic analysis of Perotin-Leonin style counterpoint he recalled reading that noted how often thirds and other intervals used as dissonances appeared in those pieces. If I can find it online, I will note it in an update. Here is Leonin, the earlier and more conservative of the two, his setting of a verse of Haec Dies, played on a portative organ. As you can hear, there are quite a few thirds, I'd argue at least a few are treated more as consonances needing no resolution.
My friend found it as funny as I did that the claim was made as it was, indeed, Christians who first began to use thirds as consonant, harmonic intervals and, as musical practice developed, began to base harmony on them.
His extensive, practical experience with the Pythagorean tuning commonly advocated in medieval theoretical and practical texts (especially the famous musical treatise of Odo of Cluny that is sometimes, somewhat humorously translated, "How to make a hurdy-gurdy" ) said that he thought the ranking of intervals as perfect consonances was directly the result of the acoustics of that tuning, based on the perfect fifth. He said that the importance of that perfectly tuned interval and its inversion to the medieval ear was obviously one of the most important and satisfying of musical experiences and that the later affection for thirds, leading to the mean-tone systems, with an emphasis on justness of thirds and a distortion of fifths was an important stylistic feature based on changing taste in music. Of course, there was a MUSICAL prohibition of the use of parallel fifths and related types of voice leading, perhaps because it was too associated with the religious practice of parallel organum. Fourths, which had been considered a perfect consonance, became, officially, dissonant.
Finally, he noted that, while it is doubtful that an unaccompanied choral group would perform strictly in Pythagorean tuning, probably singing intervals more flexibly than can be done on a fixed-pitch instrument, the sound of perfect fifths would have been thoroughly embedded into their ears and their musical experience. He said it was like musicians now having to learn to sing perfect fifths and just thirds since the slightly distorted fifths and thirds of equal temperament are the basis of our musical experience.
Now, aren't you glad I asked an expert? And that I didn't use the rather bad pun on the name of the gal who made the ridiculous claim.