I don't think we're disagreeing much. My main point, which I probably didn't express well was that running 10K in training now isn't going to do him much if any more good for ultimate than running 4K, other than the possible calorie burning implications if he's trying to drop weight.
I did a little googling for soccer endurance training, which I believe would be fairly appropriate for an ultimate athlete, and among all of the hits providing vague information while trying to sell me something, I found this page: www.expertfootball.com/training/stamina.ph
It espouses continuous running at low intensity in the 3K distance range, and higher intensity "tempo-like" runs in the 1.5K range. That was the only result on the first page that actually provided firm guidelines for distance.
You might want to look into some of these pages for a more specific approach to his situation. I don't know that running 10K is going to hurt him come ultimate season, I'm just saying that if ultimate performance is his goal, and he's not interested in being a 10K runner, he may be able to find better uses for his time, such as the weight room.