And in fact, a factory-stock Dahon Vitesse is equipped with an inner chain guide and an outer chainwheel disc. As everyone can see, on an unmodified factory bike there is no way that "t
he chain comes off the front chain ring occasionally". I encourage our OP to jump back in and try to further explain because it sure seems like the claim of "Dahon design fault" does not hold up to scrutiny.
That is not a good solution. My Zizzo Liberte came with the same setup. I bought the bike as a second from Zizzo, meaning it was either a return or a factory second. It came equipped with the same solution shown above. From the beginning, it had a tendency to throw the chain. I think it came off on my second ride. I noticed then that the plastic chain retainer attached to the seat tube was not adjusted properly and that the chain ate into it. I adjusted it properly, which is harder to do than one might think. The chain eventually moved it and came off during a ride anyway. The plastic piece now prevented me from putting the chain back on. I had tools, but not a Philips head. I was miles from home and, without the tool needed to adjust and move it. I ended up pushing the bike a few blocks to a bike shop and borrowing a Philips head from them. I removed the piece, then put the chain back on and road home without incidence. I called Zizzo and described the problem and sent them a picture of the chewed chain retainer. They sent out a new one. This one worked for a while, but eventually the same problem developed. I removed it, never replaced it. I adjusted the chain properly and have not had the chain come off in more than a year. If it does come off, I will just put it back on. I have speculated that the chain coming off may have been why the bike was a second in the first place. I had never seen a new bike come with this device before.
If I were to continue to be a problem, I would first try to address it with a narrow-wide chainring. As it comes stock, the Zizzo is somewhat geared to low.