First off, all of this internet chat about the subject is worthless. Before tendering her notice or resignation, she needs competent legal counsel from an attorney specializing in employment law. There may be federal law violations that trump Oklahoma's "right to work" laws. Regardless, proof is needed. A verbal conversation about why she is being let go is not proof.
Some employers are stupid enough to go on record with violations of employment law. Some are not. It will likely boil down to how professional their HR manager is, and how much pressure they get from the top. Either way, I wouldn't make any decisions without seeking legal counsel, even if it's just for the sake of a negotiated separation from the employer. Something as simple as a glowing letter of recommendation would be worth it.
Have I mentioned seeking legal counsel?
Some employers are stupid enough to go on record with violations of employment law. Some are not. It will likely boil down to how professional their HR manager is, and how much pressure they get from the top. Either way, I wouldn't make any decisions without seeking legal counsel, even if it's just for the sake of a negotiated separation from the employer. Something as simple as a glowing letter of recommendation would be worth it.
Have I mentioned seeking legal counsel?