What I did when I was in a similar, (not exactly the same), position, was tell the boss "sure, no problem" then tell the replacement guy either nothing, or bullshit, then when the boss wanted to know why he couldn't do the job yet, I would say "I've already shown him how to do that".
By the time they figured it out my notice was up and I left.
It really helped that the replacement was Indian and really poorly educated. He claimed to speak three languages as well as English, (which was probably true), but if his other language skills were as poor as his English it's a wonder he could communicate with anyone at all.
He was hired specifically to do my job, but there was no indication he had any experience at all. I think he lasted about 2 weeks after I left.