I have a D7 and there is no plastic piece that keeps the stem in place. There is a metal QR that secures the stem in the unfolded position. If you are talking about the plastic safety clip all it does is add an extra level of security to the metal QR - it should not be under any stress at all when riding. I would ride my D7 without it and as noted above you can make a temporary fix with a zip tie.
I do think Dahon should make that piece out of a non-brittle type of plastic. It would save everyone a lot of hassle.
He's talking about the delrin piece located inside the handlebar post, at the base of the post. It is a known problem that this delrin block can work loose rendering a handlebar post which is unable to latch. Zip ties will not work. As far as I'm concerned, it IS a design flaw, and the biggest reason to own one of their 26" folders instead. You should ask Jur about this design. He has a lot to say.
Here's a pic by BruceMetras:
white delrin block
In squikema's pic, notice he's missing the block:
no block