My point is that other breeds are fully capable of doing endurance and are used to do so. This onesy or twosey of other breeds represents that horses of other breeds can be quite accomplished in endurance and do quite well. Yes, Arabs and a few other breeds are built for endurance and, if you have one, you have a better chance of having a horse that's able to be successful (depending on your definition of success) and competitive. However, the OP didn't ask whether she would be able to be competitive - she asked
Do you think he would even have a shot at finishing?
And yes, he does. Definitely at the LD level as long as he is fit and healthy. And, if she's lucky, he'll have the motivation to. As she indicated, this horse does move, so I think she'll have a blast using him for rides. I would not be any more or less inclined to tell her that her QH could be successful in LD's or 50's as I would someone who owned an Arabian. Her horse could even be Top 10 and Tevis-worthy. At the same time, he could be none of the above. We surely cannot tell from a picture - she'll just have to ride him to find out. As far as having a shot at finishing? I'd say, with what we know from this post, he definitely has a shot at finishing.
I do agree that she and every other person new to endurance, regardless of trail experience, should get a mentor and volunteer at rides before they attempt to ride in one. Maybe even bring your horse along to see how he acts and take a little trail ride and learn how to follow the ribbons :)
I see no indication that she would enjoy ATCHA better than endurance. As you pointed out, she asked about ENDURANCE, which I do feel that LD's are a very important part of, and much more closely related than Competitive Trail in every way.
It's a bit hypocritical that you go and tell the OP that her horse is not capable of doing endurance while you do endurance with a horse "not typical" in then endurance world. You're right, each horse has to be ridden in a way that meets its needs, but that takes a lot more into account than just the breed. Like you, she wants to do endurance with what she already has, and simply wants to finish. Her horse may perform better or worse than yours, but that's simply something you cannot know from what she posted. If your horses are truly only capable of what you're saying, then there are indeed draft horses (and horses of other breeds and even TWH's) out there that do much better in endurance.
From this and other posts I've seen from you, it seems that you know some about endurance, but have a lot to learn still. Heck, I'll admit that I have a lot to learn. However, repeating the same stereotypes about endurance are what prevent people from ever trying this sport in the first place, one of which that you NEED an Arab to be competitive or even compete. The great thing about this sport is that it's open to anyone, and anyone can be successful to a certain extent as long as their horse is healthy and fit (I know many people whose best endurance horse was a grade rescue horse). Beyond that, there are certain things that make you more likely to succeed at higher levels, like going faster, placing better, and going farther, but there is no "sure" combination that's guaranteed to get there, and you'd be surprised and what things people have succeeded with.