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I agree with you, but use of 'restauranteur' seems to have been increasing quite a lot. In many cases, it will undoubtedly be an error, but in principle there's nothing wrong with deliberately coining it as a new word. Interestingly, the euphonic elision in the French original is there being replaced by what arguably is a euphonic epenthesis in English. :)
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Actually the use of the word restauranteur seems to have been in decline since at least 2004:
https://trends.google.com/trends/explore?date=all&geo=GB&q=restauranteur
restaurateur
(no n, pet peeve, nothing else to see here)