“Nothing is Certain or Impossible; there are only probabilities”

“yes that is truth but it hasn’t disproven it only made it less likely; everything is probability ”.

This person is right in the sense that probabilism is a good approach to epistemology. We don't have perfect information about most of the things we do, so taking a tentative view of most things is a sound idea. However, there are some certainties and some impossibilities. For instance - death is a certainty. Likewise, immortality is an impossibility (at least, for now).

They are wrong to suggest that all probabilities are equal, though. If, of two explanations for a phenomenon, X has a probability of 0.9 and Y has a probability of 0.5, X is clearly more likely than Y, and we should accept X over Y unless we get evidence increasing the probability of Y such that P(Y) > P(X). As it is, there is plenty of evidence that most "alternative medicine" doesn't work. Thus, it should not be seriously considered as a treatment option for disease.

So the premise "there are only probabilities" is false, and we also have an appeal to possibility in the argument for alternative medicine.

I don't know what kind of fallacy this is: besides being factually incorrect it is an outright lie and contrary to reason

In essence, you ask how to deal with the proposition: “But you can't be 100% sure that there is no possibility of X.”

I think it is essential to note that we can never be absolutely certain of anything other than metaphysical axioms, so we must assign a value to any proposition based upon the available evidence. To dismiss a proposition on the basis it hasn’t been proven beyond all possible doubt is fallacious reasoning if one seeks knowledge. 

We can, however, attain an epistemological certainty, which, loosely restated, means beyond a reasonable doubt, while keeping our minds open to additional evidence or a different interpretation of available evidence.

Knowledge is the correct identification of the facts of what exists, of reality. This is the easiest definition to grasp that I have encountered. This is an important concept to keep in mind. For example, suppose you hold a belief that does not correctly identify the facts of reality. In such a case, that particular belief is not knowledge. The purpose of the scientific method is to correctly identify the facts of reality, but beyond this discussion.

So, depending upon the context and focus of the discussion, the statement may be just an opinion or it could be in part the Fallacy of Proving a Negative and its related fallacies. 

If the argument is:

Nothing is Certain or Impossible; there are only probabilities.

Therefore, I should believe X, since it's possible.

That would be appeal to possibility 

Hi, alex!

      I’m here because, yes, you may have missed a potential fallacy. Your interlocutor’s defense against your inductive argument is to mention that the argument you gave is inductive. He may as well parry an enemy sword by shouting “my enemy swings a sword at me!” 

      Since your interlocutor takes probabilities seriously, and since he agrees with the premises of your probabilistic argument against certain beliefs he holds, he needs to show that your argument nonetheless does not lessen the likelihood of his beliefs enough to make them unjustified. He also needs positive reasons to hold his beliefs in the first place, which you should ask him about. Otherwise, his perseverance in holding his beliefs  merely because they have not been disproven is a fallacy called the argument ad ignorantiam. 

Thank you, alex.

From, Kaiden