I don't ever think a marathon is easy, but sometimes they can be not hard. That sounds contradictory so let me try to explain. Racing a marathon all out for a new PR is hard, very hard. It requires 100% effort and 100% concentration every step of the way for the entire race. Running Disney at a relaxed pace and stopping for character pictures along the way is not as hard. Maybe comfortable is a good way of putting it. It's still a formidable amount of work, but it's not hard like going for a PR is hard. In either case, running 26.2 miles is not easy, but they occupy different spaces on the sliding scale of hardness. Put another way, this is why the Clif pace team asks their pacers to run at a pace that is 1 minute or more slower than their typical marathon pace. Trying to pace the 3:30 group when your PR is 3:25 will be very hard. Pacing the 4:15 group when your PR is 3:25 will be more comfortable. I hope that makes some sense. If not, let me know and I'll try again.
On to your co-worker and an ultra, like everyone else, I'd definitely caution them against running 100 miles if they haven't done a full and a 50-mile race first. Not to say they couldn't get both of those done on the way to 100, but I think that should be a matter of years, not months. Just like a full is more than just 2x the distance of a half, a 50-mile race is far more than 2x a full, and a 100 is way beyond 2 50s. It takes a lot of time and experience to teach your mind and body how to handle the volume of training that's required to get through a long ultra safely. You can't get that experience in a few months. The process is much longer than that, and some people learn along the way that their bodies are not capable of such high mileage. I hope your co-worker gives their plan some serious thought and realizes they ought to focus on getting through a full first before they consider making the jump to longer distances.