There is one thing you can do.
Tell the person, "You do not believe this statement."
No one believes EVERYTHING they are told. So you are saying, basically, that a person tends to be gullible?
It's easy to be skeptical. You just question the validity of what is being told to you. Search for counter examples.
What motivation might this person have to develop a sense of skepticism? What motivation does this person have to apparently believe everything he or she is being told?
You want to cure someone of gullibility? Play tricks on them. Not mean ones, but ones in which the feedback is clear. The person recognizes how he or she has been fooled. Then ask, do you know how I fooled you? Then say, cause you can bet I am going to do it again. Be ruthless with your persistence and compassionate in the form that the tricks take.
Eventually, the person will develop skepticism. No one likes being made a fool.
