LEAF draws 1440 watts. No one should use a L1 EVSE on even a 12G Extension Cord or anything other than a dedicated proper 120V outlet. Any attempt to imply this is remotely safe is reckless IMO and very irresponsible. There are so many more variables to consider in the real world and it's simply a bad practice to justify even indirectly.
This assumes the worst 12 gauge extension cord, with the alternative being the best 120 volt wall outlet. A genuinely good 12 gauge cord plugged into a non-worn outlet will barely get warm. So rather than tell people not to do things they will do anyway, I suggest you tell them, as I do, to make sure the cord and outlet are both high quality and in excellent shape.