The 4 hour 140* "rule" is for fresh sausage and meats. They have no cure to help them bridge longer time periods and be safely cooked. Since he used cure, exceeding 4 hours would have not been a problem in the first place. With cure you are using the cure to keep the bacteria from growing instead of just heat like in a natural uncured sausage. So I'm sort of on the fence also as to should he toss what he has. In theory it should have been safe through the point he pulled it early. And if it is treated as "fresh" or uncurred sausage on the final cook to come, would it not still be safe? Part of me says "should be" but part of me is skeptical also (yeah, not much help here, I know).
I would probably thaw it in the fridge, and then grill to finish. I WOULD NOT continue to smoke it, but hot grill. Might no be the USDA correct answer, but I'm loath to throw out meat unless I'm certain something is wrong. Especially if I'm the one doing the eating. I would not use this for a gathering though.
And not to confuse thing even more, but in the store you will see the phrases "natural uncured" on sausage and meat packages. Usually that is just a term meaning they did not add "cure", but used celery juice which contains the exact same ingredient in cure #1, just in smaller quantities. It's a marketing trick where they charge you more for what some perceive as a healthier product. When the fact is nitrates & nitrates are naturally occurring compounds in a lot of food items. There is more in lettuce and other green veggies than sausage. The only difference is the "cure" comes from crushed plant matter vs cure that has been distilled out and mixed in a precise amount with a carrier such as salt (as the cure part is a very small percentage of the cure #1, so adding a carrier such as salt makes it much easier to measure). Not a thread hijack, but when discussing cure & the 4 hours rule this usually also comes up (the part about "uncurred" meats).