A new, low time multi engine pilot and an instructor with zero in type? Yeah... Good luck with the insurance.
I tried the same thing in a Duke a few years back. I had my ATP, about 3000hrs, 500me almost all of which was PIC and a background of teaching ground schools on spin awareness that focused heavily on multi engine airplanes(which happens to be where I met the owner). I had tons of piston and TC Beech time, Bonanzas, Travelairs, Barons as well as 310's, and a little Navajo... I even THOUGHT I could fly anything. They still wouldn't even let me look at it because I had no time in type and flying it with the owner did not count.
If your student really likes flying with you, then you should go to training together on the airplane and he should pay. That way, the two of you have the same foundation to start with. You need to be able to dig yourself out of a hole faster than he can throw the dirt back in it. Also, who's going to be PIC? Are you going to act as a crew or keep the student/instructor relationship?
$400 ain't bad for a day of baby-sitting the owner. However, any qualified Cheyenne instructor should be getting at least double that.