Yes, it certainly is possible for someone to get confused and mix things up if they spend too much time listening to false doctrine or hanging around with false brethren. What it comes down to then is: what did they believe at the time they professed to come to the Lord for salvation? If they believed in works then, they never were saved. If they truly believed the Gospel according to the Scriptures at that point in time, then they are saved - they unfortunately are seriously confused/deceived and need to spend some major time in God's Word sorting out what the Bible says - not what their cultic friends have taught them.