Have some tact. This dragon`s tank was not filled with sand, it had a small amount of sand in one area of it`s tank. Why? Because some beardies like to dig and are picky about where they sleep(I have a sand box in one of my beardies tank because she is a big time digger, does that make me a bad owner?). It doesn`t make someone a bad owner to appease their animal like this. She obviously cares very much about her animals very much so seriously, lay off. Your behaviour is shameful and even if her beardie did die because of some fault of her own(which he didn`t) you have no right to make her feel bad, losing her animal is bad enough. Condolences or GTFO.
I`m really sorry this happened but it`s like I said to Kitsune, sometimes if an animal is distressed for some reason they will do strange things(much like dogs eating grass so they can throw up.) It probably has nothing to do with the sand at all. Maybe he was in pain and stuck his head in the sand or gobbled some up, animals just sometimes do things like that when upset or in pain.