How do I deal with someone who refuses to learn from scholars, slanders Salafis, and visits websites that are abusive towards Islam, knowing that he prays? And what is the ruling on my describing him as "on the verge of disbelief"—is that considered backbiting?

A Muslim should advise his brother and supplicate for him, and seek out righteous company for him. As for your friend, his consideration of the Qur’an and Sunnah as references is commendable, and he should be alerted to the fact that the words of the people of knowledge assist in understanding them. Try to show him the words of contemporary Sunni scholars and explain their eagerness to act upon the Qur’an and Sunnah. Regarding his exposure to the Christian website, explain its danger and its prohibition for those who are not firmly grounded in knowledge, and clarify to him that the inability of the ignorant to convince Christians does not mean that Islam is not the truth. As for your conversation with your friend about this person, if it was motivated by compassion, then there is no sin upon you, and there is no harm in asking for forgiveness without explicitly stating what you said.