Personally I think if my dentist has sent me a similar letter 15-20 years ago I would have 1) not believed him for a second that I might have celiac and 2) freaked out cause it would be scary to get a letter like this.
However, I will say that if he examed my mouth/teeth and then verbally told me his findings it would have gone over much better. He could have answered questions I had, reassured me I'm not going to die within the week, given me a print out, offered to write a letter to my doctor, and also told me that chances are I do not have celiac but I should perhaps be tested because there is a slight possibility of it, and it would be good to know this sooner rather than later so I can start treatment. I think if any medical doctor/dentist sees something that is not in his field of expertise but is concerning he has an obligation to inform the patiet (for instance if the eye doctor notices a bad looking mole on the tip of a patient's nose he needs to tell the patient to get it checked out and follow up to make sure this happens, unfortunately most doctors don't have time to make sure the patient follows up.)
I remember going to a new dentist 15 years ago and the hygenist commented on something about my teeth, I don't remember exactly what it was other than some sort of abberation such as ridges or pitting or something that I now know can be indicitive of celiac but back then I hadn't a clue. This along with a lifetime of multiple cavities, root canals an crowns should have clued a dentist in somewhere along the line that something might be going on but other than this one hygienist there has never been any mention of anything at all by any other dentist or hygenist. Probably most dentists are as clueless about celiac as other doctors. SInce my siblings have similar dental issues but do NOT have celiat it may just be a genetic thing.
(As an aside I just did a search of images of "teeth celiac" and I've got to say my teeth don't look like any of those. IMH, I think the vast majority of teeth problems are not celiac related)