IMHO, as an owner, operators of a machine, especially excavators, should be aware that there is times that the machine can be placed in a position that damages it. Many types of machines are able to do this. The likelyhood of this occuring is increased by quick couplers, and non standard buckets or attachments. This DOES NOT, in my opinion, make it excusable for an operator to place the machine in a position that it causes damage.
On an excavator in particular, the interferance is located in front of the operator, and within +/- 3 feet of his eyes. There is no reason for damage like this other than a lack of attention by the operator. The only exception I could see to this is if a failure of the hydraulics let the boom, stick, or bucket fall, and from your description that is not the case. It is also very unlikely. I have never seen it happen in 24 years of experience.
In my opinion, your friend is likely to recieve a reprimand of some sorts, depending on company policy. I would do anything from a strong warning if it was a first offense and he was a good employee who just slipped up, up to firing, if it was yet another in a long string of accidents. Employee attitude towards what happened will have an effect on the severity as well. Trying to make it to be someone elses fault, or the machines fault, instead of owning up to making a mistake, would not sit well with me.
That's my $0.02 worth. Take it or leave it.
