Just a thought, but have you considered attempting to get in touch with someone like Dr. Tobias Capwell, or any other person of scholarly authority who has knowledge of arms and armor and asking them to send off an e-mail to this professor? You're not just trying to convince some friend off a misconception, but a fellow historian in a position of authority and instruction who could spread the misconception to many others. Dr. Capwell might think it worth his time to try to set this professor straight.

I was once on my way to give a presentation in one of my history classes at college while in full harness. While waiting outside a passerby commented on how heavy it must be. I told them it really wasn't all that bad. They asked how I could even move in such a heavy suit. Again, I let them know it wasn't excruciatingly heavy, and even did a few jumping jacks and pushups to demonstrate. They somehow still maintained that I would be unable to get up if I fell over.
Just goes to show some people will hold onto their beliefs no matter how in-your-face the evidence is against them.

...what about this?;

http://vimeo.com/5169999

Cheers, Timm

...or even better:

http://vimeo.com/5130297

https://vimeo.com/5131556
small child on one side of a teater totter, maile and padding on the other end