As all the other posts have mentioned, this sounds like a tough situation where someone's hiding behind the "reality might not be real" routine in order to avoid facing the facts. I really like all the advice given above.
As others have noted, her stance is circular and self-defeating. By even responding to your arguments, your friend has affirmed that (a) what you are saying to her in your debate is real, (b) that she knows for a fact you are really communicating with her [otherwise she wouldn't bother responding to you and claiming "everything might be a hallucination"], and (c) that it's therefore possible to know the truth, which refutes her entire "it's impossible to know anything for sure" premise.
If you want to take a satirical approach, you could tell her that you have no idea whether her arguments are real, since you might be nothing more than a brain in a jar with mad scientists around you inserting delusions of philosophical debates into your synapses—as such, you cannot possibly be bothered to respond to anything she says, since it might be a hallucination resulting from a crazy science experiment.
Further, by maintaining her "reality isn't real" and "truth is impossible to know" stance, she's unwittingly claimed she personally cannot have anything verifiably valid, truthful or otherwise relevant to say—in which case there's no point even listening to her. Granted that might be a pretty blunt approach—not sure if she would respond to that direct of a reality check, or if a softer approach should be taken. I'd depend on her personality. Best wishes with connecting with her!