There is a school of thought that says THC and that sort of stuff can reduce the effectivemess of the local anesthetic. But then you'll probably be so out of it you won't notice.
But on a more serious note if you need to get youself knocked out (FULL anesthestia) the dentist is not qualifed to do that (although that never stopped them when I was a kid and was regularly knocked out by the (unqualified) dentist) so they will have to get a an anesthesiologist I would imagine.
I have plenty of work done at CM Ram (with local anesthetic) - my advice (for your friend)n- go ONCE get all your teeth pulled out and then get some plastic choppers for the rest of your life.
Apparently this was quite common before free dentistry "It may sound gruesome but as late as the 1940's, people were having all their teeth pulled out as a birthday or wedding present. In the days before the NHS, no teeth = no expensive dental bills."
https://www.bbc.co.uk/scotland/history/scotlandonfilm/forum/health/thread10.shtml#:~:text=It may sound gruesome but,teeth %3D no expensive dental bills.
Problem solved.