david - firstly I think most here would agree with the fact that as hard and horrible this accident is that it sounds as if you provided him with a high level of training, accidents do happen and I don't think it helps anyone beating yourself up about what more you could have done.
Andrew seems to have had an excellent reputation as both a top bloke, and a fantastic pilot, and I don't think anyone will ever take that away. I think more to the point and 'spirit' of these threads is for everyone to try to understand what happened, and IF mistakes were made so that others can learn and put these lessons away in their toolkit so that if a similar situation happens they can perhaps produce a different outcome.
We may be pilots, but firstly we are HUMAN, and sadly humans do (and i'm NOT saying he did) make mistakes. There are literally hundreds of highly skilled pilots that are no longer with us, that sadly in the heat of the moment made mistakes, does that make them bad people not worthy of our sadness of their loss - hell no - but lets not let their ultimate sacrafice amount to nothing. I know a few of them no longer with us and I know them well enough that if you could talk to them now they would be saying 'why did I do that, I should have done XXX', hindsight is an amazing thing, it can't help those involved but it MAY help others avoid it after learning from it. Learn from the mistakes from others, because you will never live long enough to make them all yourself ! Isn't that the purpose of accidents investigations ?
Something no one else has mentioned is that there is no shame in talking to professionals about grief in these situations, it doesn't make you less of a man(or woman) to talk to someone, it's never easy to cope with the loss of a family member or close friend, there is no SOP for grief, some people deal with it better than others.