I hate to say it, but I think your buddy should stay home.  He has an achilles problem that required surgery and it didn't work out.  Now he has a second surgery planned and wants to walk over rocks, roots and be part of a crew on a wilderness canoe trip miles from help.

What will you do if his achilles blows out somewhere near Brent?  If I were your buddy, even if I felt I could do the trip with minimal risk, I wouldn't put the rest of my group in the middle of that decision.

Better to go with an odd number and rent a solo canoe or 3-seater than risk injury and ruin the entire group's trip.  There is always next year when he'll hopefully be healed 100%.

My two cents, of course.