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The process for getting a green card through marriage takes years, about three to five. During this whole period the letter writer would have to pretend to her family, friends, colleagues, acquaintances, everyone that this is a legitimate marriage. She'll have to live with this man during this whole period and get joint bank accounts and joint mortgages/leases, etc. Her sham husband will have the full legal and financial rights of a spouse, which will affect the letter writer's personal and business decisions and choices. This "sham" marriage will be a life-altering event, in personal, practical, and legal terms. And it might be even harder than a real marriage, because she'll have to do it without the emotional commitment and goodwill that a real marriage comes with and she'll have to do it with the constant possibility of being caught.
It would be one thing if she's planning to do it with someone whom she already knows well and has worked out the details (hey, but people are people, and often they don't stick to their agreements); that would be hard enough. But I should think if she's considering to go through this with a complete stranger, then it would be nearly impossible to pull off and still have a normal life.