I asked some questions but did not get a response to those questions, so I will try to answer your question without the information that I requested.
Her employer is correct that an "at will" employee can legally be terminated without cause. There doesn't need to be good reason for the termination, or even any reason at all.
Now, you noted that she is alleging that this termination is actually based on retaliation and discrimination. Those two terms have very specific meanings though and your facts don't fit either.
Retaliation requires that an employee be terminated based on them having made a complaint about race, religion, gender, age, disability or FMLA use discrimination, OSHA complaints, wage and hour complaints or worker's compensation complaints. It is not illegal retaliation to terminate someone for letting an employer know that they are considering work elsewhere. That is actually a legal basis for a termination.
I asked you what you meant by discrimination, because these facts don't show that either. For there to be discrimination, you have to show that a decision was made based on a person's race, religion, gender, age (over 40), disability or recent FMLA use. Without those factors, it is not discriminatory to terminate on these facts.