Sorry to hear that.
I give your buddy lot's of props for even giving it a try.
Paddling even when healthy at a hard pace is hard on even a healthy back. As someone who has dealt with back issues, I can only imagine doing in a canoe marathon.

When I checked your progress this morning, the GPS site said the average speed was about 8 mph, but your "current" speed was about 1 mph? I just figured you had stopped for a minute for whatever reason.