It will help to know a bit about the cult itself. Remember that those cults often use names and words familiar to Christians, so it will be important to define terms. Otherwise, it could seem as if we and the cult members are "on the same page" when indeed we are not.
As the question is how to counsel someone that has been approached by the cult group, it may be possible to be very direct and open about the major differences, and to point out the main points of contention. For example, having dealt with Mormonism for many years, I am aware that their doctrine of God is thoroughly unbiblical. A statement made by one of their leaders years ago can serve to illustrate. "As man is, God once was. As God is, man may become." Latter-day Saints (as they now prefer to be called) still hold to this teaching.
My suggestion, therefore, is to learn something about the major teachings of the cult in question, and share with the friend or relative how the cult is in direct conflict with the Word of God.