If I understand correctly, you wish to abandon the return trip for 2 of the 3 passengers on the reservation. I don't believe this will be any problem at all for Copa. Maybe they will ask for the other two passengers at check-in, but all he has to respond is that they will check in later. They won't yank your husband off the plane and demand he pay a fee or cancel his ticket. You might consider calling Copa and separating the reservation, but I can't think of any scenario where that would be beneficial, except for some sort of irrops or something where the unattending parties cause a problem some how.

Disclaimer: I only have one experience with this. My wife once dropped a return trip in favor of travel to a 3rd city. No repercussions whatsoever.