They are correct ...
If that FQ has been run without a cone, then the secondary filter under the cone guard will need replaced. It is more than likely blocked, and that will cause the motor to overheat and eventually burn out. With the Filter Queen, If you use the filter cone, and never run it without one, that secondary filter will last a long time, but it still needs replaced periodically. I have a FQ Princess II that I use as a daily driver, and I replace the cone once a month, when I empty the canister, more often if I have to vacuum finer dirt, and replace the motor filter every six months without fail. The internal motor filter is easy to replace. If you look at the manuals page on this web site, It shows how to do it. As was stated by others earlier, most vac shops should carry FQ cones and filters. If not, Genuine FQ maintenance kits are available on eBay for reasonable cost. They include 12 cones and 2 motor filters. This is enough to run the machine for 1 year. I would not use it again until you have both a proper cone, and clean out the motor and replace it's filter as well. I dont have a spare at the moment, but will be ordering again soon. Let me know if I can help you find the proper supplies, and I will be happy to help.
Joe