What was Kalanithi's support system? Who did he depend on, and how did he react to this dependence?

Dr. Paul Kalanithi, neurologist and author of When Breath Becomes Air, died of metastatic lung cancer before finishing his memoir. Knowing that he didn't have much time left, Paul asked his wife, Lucy, to help him complete the manuscript so that it could be published. (She did, and she wrote the epilogue to the book.)
This fact is key. Even before Paul realized he was sick, Lucy was a strong pillar in her husband's support system. She's a physician as well—they met in medical school—and so, in addition to being his wife and partner, Lucy had the educational background and experience to grasp what Paul's diagnosis meant and to help him make practical choices about how to deal with it. It's her medical knowledge, in addition to the love she feels for her husband, of course, that makes her a strong figure in this difficult period. She's a doctor already: she already has people depending on her, day in and day out, so she's comfortable accepting more responsibility in her husband's life as his health falters.
Dependency is an interesting theme in this narrative. Together, even as Paul is growing more dependent on his wife, they decide to take on another big responsibility. They decide to have a baby, knowing that he probably won't survive long enough to see a child grow up. (This is one of the central questions that the memoir raises: What makes life worth living? Is it still worth it to try for the things you want, even with the full awareness of your own mortality?)
Lucy is perhaps the most important figure in Paul's support system: she's the only one who's with him when he dies. But there are other important people in the book we can mention. 
Paul writes about his support system at the hospital. He's grateful to his own doctor, who suggests skipping chemotherapy for an alternative option—a pill, Tarceva—which, at first, works well and allows Paul to return to work for a short time. Paul also has support from co-workers. He recounts a conversation with his co-resident, Victoria, and how she supports him in his decision to step away from some of his responsibilities at the hospital. 
Finally, we should mention Abraham Verghese, a fellow physician and writer who helped Paul to navigate the literary world before his death, paving the way for the posthumous publication of his book.