Yes
Leadville is much less technical trail running than many of the 100M races
Its elevation change is nothing special
But the elevation that the race takes place at 10,000-14,000 feet is the real challenge.
I have run shorter trail races (50 milers) with lots of elevation change (> 10,000 feet)
I have also run timed races where I have went further (147 miles / 154 miles) in 24 hours
But this will be my 1st 100M trail race - I signed up yesterday. I have my pacer lined up. I have the worlds best crew - My Wife. (Note usually family makes a bad crew - But she does not mind seeing me suffer)
It's gonna be fun!
PS - I think you are assuming that because an athelete is a Clydesdale that he/she can not be competitive - I am running Leadville for fun, not as a goal race. The only way I will not finish well under any cut-off is if I get hit hard by altitute sickness, or if I get injured for the race. I typically run 5,000-6,000 miles per year, so the 100M distance is not really an issue.
I am not sure what your friends situation is, but skinny does not = high level of aerobic fitness. Skinny also does not = fast.