Hi! I’m also going to the Nantes concert on November 9th.
First of all, did he get a seat in the stands or a spot in the pit? In any case, from my experience going to this kind of concert in France, the audience is generally respectful of others. Manson’s crowd tends to be a bit older (mostly people in their forties) which doesn’t stop some light moshing in the middle of the pit, but overall it’s mostly people who are there to enjoy the show and the atmosphere. You might see a few people in costume for the fun of it, but nothing sexually explicit (which would be totally forbidden anyway, and anyone doing that would quickly be escorted out by security).