I am so sorry to hear about your friend. But I think the wife probably does not have much of a case. This is a terrible tragedy, but I do not think there is any way, ethically or legally, to hold the company responsible.
Many states have “at will” termination policies—meaning you can be let go at any time, for any reason. (The only exceptions to this are things like protected groups—e.g., you can’t be fired for being black.) This is in fact legal.
But it sounds like you’re not asking about suing for wrongful termination. It sounds like you think his wife should sue the company because he killed himself. And I cannot imagine any situation in which this would hold up in court. Again, this is tragic, but it is not the company’s fault that he chose to kill himself. They could not have known that would happen, they could not have prevented it, and even if they could, it seems unreasonable to require employers to keep employees on because they’re afraid of what the employee would do if let go.