Gringo, I defer to you. Square knots do have that unhappy ability to untie themselves. My little climbing was long ago and far away. So, tell him how to do it, and save the $200 on hardware.
I'm not sure I'd call prussicking up a mast unsafe though. If it was a short clear run--and his is short--heck, in a slimmer younger life, I've used them to go straight up street lamps that weren't tapered a whole lot more than many masts are.