Does the speaker's agreement that "Salafis follow Satan"—while knowing with certainty that Salafism is the correct understanding of Islam—make her an apostate?

You did not commit an act of disbelief (kufr), but you erred in condoning the woman's false statement. You should have advised her gently and defended the honor of your Muslim brethren, as the Prophet, peace and blessings be upon him, said: "Whoever defends the honor of his brother in his absence, it is a right upon Allah that He frees him from the Hellfire." We advise you to clarify the truth, not to condone falsehood, and to advise those who err, as long as no greater harm results from it.