It is unfortunate whenever you have a bad experience with a volunteer, but it is a life experience none the less. It is hard to say why the volunteer reacted the way they did. She may have had a bad day. She may have been a parent expecting to get a field reset job and instead was sent to be on safety glasses patrol.
At the Volunteer coordination meeting right before things opened up, they reminded us that have volunteered before to be as cheerful and positive as possible. They also reminded us that many of the volunteers may have never volunteered before, and to try to keep an eye out for this sort of stuff.
I personally would hate to be the safety glasses person. Not because I think handing out glasses is boring, but because of the thousands of people coming through the door, many do not have glasses with them. A fair amount of those people like to argue about the necessity of "needing" safety glasses.
My guess is this particular volunteer had a very strict view on who was to receive safety glasses. This coupled with a few arguments on the need likely lead her to be more abrasive than desired.
You did the right thing by reporting the volunteer. On the outside, we can only guess if/or what action was taken.
The brain is an interesting thing. Often conflict will inact a
fight or flight portion of your brain that can lead to irrational behaviour. Some folks are more prone than others to have this area tripped. Being the "Safety Glass Police" likely encounters a lot of conflicts/stress and she was probably in that state of mind most of the day. Learning how to recognize when you or someone else is in this state of mind and how to counteract it is a great skill to master. As a mentor you will encounter many many many more situations similar to this, and your ability to deal with these situations will be a role model for your students.
I know when I get that way, my face will get warm and red. It doesn't happen often, but still more than I would like. You might have some sensitivity over the "real mentor" comment (which is a pretty rough comment), and may have been more argumentative than you realize. That doesn't excuse the other persons behaviour, but I hope it might explain it a bit.