39 year old man with a spot on his lung-what could be the possible causes be?

My friend, who is only 39, has a spot on his lung. He is the military and has been deployed to the Middle East several times. He is not a smoker, but does dip/chew. Also...before he joined the Army, he smoked a lot of marijuana, but that was a long time ago. Any ideas about what it could be?