The one thing I have learned over the years of doing adventure races, marathons and ultramarathons with friends of mine is that you have to keep things brutally real from the onset. If you two are supposed to hike together, stay together and count on one another if things unexpectedly go south then you both are obligated to be in the best possible shape you can be in barring unforeseen circumstances like a training injury. You have been doing your part to get ready for this adventure and your partner should have been doing the same. Runners deal with this all the time when you get someone who wants to train with you to run a marathon but won't put in the hard work to stay at the agreed upon pace and then on race day expects you to stick with them at a much slower pace. Tell your partner that the hills won't get any less steep and the pack won't get any lighter no matter how much he doesn't train. After this trip I would choose your partner(s) very carefully the next time you agree to spend the time and money on the next grand adventure. Best of luck.