Since this is an endless argument I've seen repeated in numerous MTB and roadie forums...
My choice to wear headphones is mine and mine alone. If you do not like it, too bad!!!
I wear them, in both ears, and the volume is pretty friggin high. I'm still able to hear things. I can pick up just about any non-natural sound (gear shift, chain-slap, cough, brake squeal, talking, etc.) In the three years I've been doing it, I have been spooked by humans maybe 5 times. None have resulted in an incident.
All riders are expected to be riding under control of their speed and able to respond to other trail users. Sorry to read that someone was injured but both riders have to accept some level of responsibility for what occurred.