Wow - horrific news, and a reminder perhaps that diving can always be a risky sport, and that especially during initial training it is critical to keep a close eye on students, and ensure they are well briefed.
Further to what vicko said above - this really is the cruisiest of sites, and the conditions yesterday were just fine for diving there, if slightly grey and rainy. This would have brought vis down a fraction, but I would still imagine it was at least in the 4-5m range.
Given her age, I would imagine that heart attack is unlikely. As ever with these discussions, speculation is only of moderate value, however do people think that a panic with the reg getting knocked out/pulled out deliberately and ingestion of sea water would be the likely situation?
I would also add that the dive shop involved is an outfit I've dived with numerous times, and know the boss pretty well. I've always found them to be pretty good and attentive to certified divers; their equipment is always in excellent condition; they have a very cautious attitude to the selection of dive sites (which I find a bit frustrating sometimes, I must admit!!). I haven't done any training with them, so can't speak for that, but know they are a good, safety conscious outfit.
Commiserations to the divers family and friends, and to all those involved in the dive who witnessed such a horrific incident.