Good Luck on this sometimes it's hard to be so nice, Be Gentle and do it on the phone, that you've thought about it and really don't want to do the kit, blame it on creative process because a project this large you really need to enjoy, Let her know you think the world of her but just one per person/couple, you have others many still to go!
I'm sure she realizes hubbie can be a pain, let her know don't want to do one for him, may seem mean but could completely solve the problem, she probibly just ask for him. May have thought no labor would be applied since they bought the kit.
Back to Good Luck :)