I stand by my previous comments. Have a conversation, then take it from there. In this particular case, it's clear that you don't think Vortex is a "safe" recreational cavern/cave (I have never been there to have an opinion one way or another). Since you have had the conversation, and you think he acted inappropriately, if I were in your shoes and I felt that strongly, I'd figure out how to "take it from there" - with my end goal in mind the whole time. I have to caveat this whole thing, however, by saying that I seriously doubt you are going to change this guy's mind through a letter.
OW divers dive in "safe caves" or "caverns" every day in FL at the Ballroom at Ginnie Springs and at Blue Grotto... these are two places I can speak to, with which I am familiar. (Let's not even start on Mexico) I will argue every day and twice on Sunday about how OW divers need to stay out of the Ear and the Eye at Ginnie, because it is SO EASY to get in trouble there, but personally I am not going to argue about the Ballroom or about Blue Grotto. Heck, I'll admit I went into the Ballroom before I was cavern certified - someone can just slap me about the head with split fins now!
My thought is that this instructor thinks this room at Vortex falls into these categories.
And I would be willing to bet there are an *awful* lot of OW instructors taking divers into Vortex... as discussed, the options at this point are to write a letter (which I believe would fall on "deaf" eyes), publish his name on the boards (I'd be careful there, libel laws are interesting - what you saw on that video and what *I* saw on that video are NOT the same <G>) or to report him (which would address liability issues but IMHO not do anything in the long run bc there would be an awful lot of instructors in trouble for taking people into "safe" overhead environments.) I'm not being very helpful am I?
Let me ask a different question.. what is the end goal? I mentioned this earlier - I wouldn't do anything unless I think it is ultimately going to help accomplish my end goal. I am assuming in this case your primary goals is really to get this guy to stop taking divers into the cave at Vortex... If so, I'd be looking at this slightly differently, given that none of the above-mentioned options would accomplish that goal.
What could? Maybe - one thought would be talking to someone HE respects and who has the same opinion about OW divers in the cavern/cave Vortex, and seeing if THAT PERSON can convince him... THAT may get something done. That is one thought off the top of my head, I'm sure there are better ideas.
Ah well, enough on this subject. Clearly, I need to get work on laundry and bill-paying and stop using SB as a procrastination vehicle/mechanism!