The video is less clear than the photos that he posted on the forum, which together with his description do suggest a vortex at least. And the photos also seem to indicate a connection to the cloud base, so I don't think it's a gustnado either. So I now think it was a very weak tornado.
Thanks, I made a radar animation with the white marker at this position. To me it looks like a quick spin-up happened along the squall's gust front when new cells initiated on it. https://streamable.com/4omrus
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The witness was located exactly there : 43°10'32.3"N 2°21'14.3"E.
He says it was between 16h07'45sec and 16h08'05sec.
He will search today for potential traces of this phenomenon.