In that case, the law does not typically require that the employer pay any severance to employees.
Because of this, your wife would not normally have signficant leverage to negotiate more severance unless there is a basis for her to allege that the employer has violated the law in terminating her, such as violations of anti-discrimination or anti-retaliation laws.
If there is any wrongdoing she can point to by the company with regard to her employment, then that should be raised in attempting to negotiate a higher severance. Otherwise, unless they are willing to negotiate, they may rescind the offer of severance altogether.
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