Assuming this was not a government job and that she was not a member of a union with a collective bargaining agreement (CBA) with the employer then the employer may fire her for any reason it wants other than a few reasons prohibited by law. The prohibited reasons include firing you because:
- of your race, color, religion, sex, national origin, citizenship, age, disability, or genetic test information under federal law (some states/localities add a few more categories like sexual orientation);
- you make certain kinds of reports about the employer to the government or in limited circumstances to specified persons in the employing company itself (known as whistle-blower protection laws);
- you participate in union organizing activities;
- you use a right or benefit the law guarantees you (e.g. using leave under FMLA);
- you filed a bankruptcy petition;
- your pay was garnished by a single creditor; and
- you took time off work to attend jury duty (in most states).
The exact list of prohibited reasons will vary by state. No federal law or Oklahoma law expressly prohibits firing an employee because the employee or the spouse of the employee was arrested for a crime. However, in some cases a policy of firing based on arrest or conviction records can result in violating the law against discrimination based on race. You’ll find the federal Equal Employment Opportunity Commission (EEOC) position on that here: http://www.eeoc.gov/laws/guidance/arrest_conviction.cfm
I think she’ll have a hard time proving that the employer was using this as a pretext for firing her for some protected reason like her age, sex, etc. The employer hired her just four days before. That suggests her age, sex, etc., wasn’t a problem for the employer. The employer then found out about the husband’s arrest and fired her. If the reason really was that — the husband’s arrest — that’s not illegal and about all she can do is file for unemployment and look for a new job.

