Fast travel to Dr. Earnheardt's mansion and head to the cave. After the scene, you can talk to each of your friends then exit the cave.
Fast-travel to the Amanaki Outpost and head to the pen to contact Willis. Next, race to the airstrip before the timer runs out. When you're near in the vicinity, the timer will disappear and you'll have to take out the first wave of pirates attacking Willis.
Next, you have to dig in and protect Willis as he repairs the plane. It will be a good idea to place some mines around the houses. There are some supplies and a mounted machine gun in the middle. Take out the enemies as they come until Willis takes off, taking you with him.