From the civil rights act of 1964:
"Expanding Rights under the “Retaliation” Provision of Title VII. Title VII of the Civil Rights Act of 1964 (the “Act”) prohibits an employer from retaliating against an employee who has “made a charge, testified, assisted or participated in” any charge of unlawful discrimination under the Act."
http://www.sgrlaw.com/ttl-articles/826/
If the civil rights act of 1964 prohibits something, I think that that fits the common definition of "illegal" that a normal person would use.
Maybe a lawyer would use a different word, but who cares, everyone understood what you meant.