AlteredState I am going to have to disagree with you.
First, glucagon secretion is not absent in the type 1 diabetic (T1D). However their glucagon secretion is very abnormal and does not provide the T1D with the same capability to release glucagon in large quantity to reverse hypoglycemic events as that of a non-T1D person. The system is dysfunctional. The exact reason has yet to be determined but I believe it is because the glucagon-secreting alpha cells of the pancreas lose cell-cell communication with the insulin-secreting beta cells during T1D pathogenesis and it sends the glucagon response haywire.
Back to the runner. Any time training session can be hard for a T1D to manage, if they have been administering insulin doses (especially larger ones) in the few hours directly before said training events. However this is no reason to suggest not to do it, the T1D (ie, the nephew) just needs to monitor his glucose level before exercising and think about what insulin he has taken that day (short acting and long acting) and plan accordingly.
Obviously it is a bad idea for a T1D to eat a large meal with an estimated carbohydrate content (think, a fast food meal), take an amount of insulin that should cover what was eaten but could be slightly too much or too little, and then workout 30 minutes later expecting everything to go smoothly. I highly prefer to not have taken any insulin 2 hours before any work outs, else I am very paranoid and will cut workouts short if I start to feel excessively tired or lethargic during the workout.
The same is true for a long running event. Just make sure your nephew doesn’t take large doses of insulin, especially fast acting insulin, in the 1 hour preceding his race. If he wants to eat immediately before a long run, I suggest high protein, low carbohydrate options like protein shakes, nuts, low carb protein bars, water. Tell him its ok to eat some carbs before he runs, but don’t overdo it (think 20-30 grams carbohydrates MAX without insulin, or maybe 1 unit fast acting insulin). It’s always better to be high than low during strenuous activity, although it is optimal to be in between and not high (>180 mg/dL) or low (<70 mg/dL). If he plans correctly, he should be fine.
Odds are, if he is running distance he will not want to run with a pump attached so this is probably out of the question and he will need to go solo (as I do 24/7) for his events.
But do not let this talk of coma and death scare you! Proper knowledge and preparation are all that is needed. This is not to understate the challenge, because it is very hard to do control everything perfectly, but do not discourage him. Believe me, one of the first things to happen if your nephew goes hypo during a race are that he will lose his ability to focus on running, lose his desire to exert energy, and probably get weak in the knees (like I do). This will happen long before he passes out or goes comatose…
I have been T1D since I can remember, 20+ years now. I also just saw the figure competitors photo with the blonde & the apple. Jesus… now what were we talking about?