this is interesting in that a similar thing happened to william and mary a couple of years ago. in that case, the W&M runner didn't walk/jog to the finish to just be the last finisher and give W&M an official team score. as a result, W&M didn't have a team place (if i remember correctly they would have been in the 20th-25th range if you just tacked on a last place finisher).
i tend to feel like it's fair to simply allow a team to tack on a last place finish when something like illness or injury would otherwise keep the team from having an official score at nationals. that didn't happen for W&M though, so if the georgetown runner really did leave the course, it seems like it would be inconsistent to allow an official team score for georgetown. at the same time, the W&M runner a couple of years ago didn't actually walk across the finish line so there is a difference.
one last thought - it does seem like the NCAA would not want to force injured athletes to walk a mile or two just to be the 5th scorer on paper. so i would support an official policy allowing a penalty score of last place to fill out any team's scoring. it's just too bad for W&M that such a policy was not in effect a couple of years ago.