As his colleague, you can plumb your professional network for opportunities and talk up what you know of his skills and work ethic.
As a mentor, you can offer to review his resume.
Most important, you can coach him to advocate for himself with management: "I can't afford to stay here at my current level" might work, but it would be more effective to illustrate why his performance merits a raise — or truthfully mention the tempting offers he's considering from competitors.
And if all else fails, his briefly retreating to the parental sanctuary, assuming it's safe and supportive, might not be the worst outcome while he regains his footing. If nothing else, it frees up mental bandwidth for long-term planning that would otherwise be spent pursuing things most of us take for granted, like a hot shower.
Karla L. Miller writes an advice column on navigating the modern workplace. Each week she will answer one or two questions from readers. Miller has written for and edited tax publications for 16 years, most recently for the accounting firm KPMG's Washington National Tax office. You can find her on Twitter, @KarlaAtWork.