I think i've trained guys on a little of everything. From Sticks to duraplanes, LT 40's, debs, and lots more. Of the lot, the duraplane trainer is the worst. Its too heavy to land well. I've even made larger, lighter coro wings for a couple of duraplanes, and its made a huge improvement in how they fly.
As far as the Deb goes, there's nothing wrong with it as a trainer. Its not as floaty as some of the wood trainers, but floats much better than the Duraplanes do. The deb handles wind much better than the average balsa trainer does. Its stall speed isn't quite as low, but, its not high enough to be a problem.
Ask around, there's probably more than one instructor, and some are bound to be more open minded than others.
The only key ingredient that i've found in teaching someone to fly, is that they have to want to learn. The plane is nothing more than a means to an end. If they want to learn, they will, no matter what the plane is.
I think i'd build the deb, and let one of the instructors fly it, and see what they think. Chances are, they'll be pleasantly surprised.