Chess: c5 as opening move for black?

I have an acquaintance that is a pretty good chess player. He is not a Master, but I think he is called an Expert. Anyhow, against every first move for White he answers with Black the first move of c5. His opponents have opened 1) e4 1) d4 1) c4 1) f4 1) Nf3 1) Nc3 and I think even once an opponent of his opened 1) b4 but in every case he responds as black with c5. I asked him a bit about it, and he said something about Sicilian defense and Benoni Defense? I then asked him had he ever lost his c5 pawn and been down a pawn? He said in many openings the c5 pawn is a gambit pawn? Anyhow, I don't know him very well, so haven't gotten into great detail about it. But is anybody out there an authority on c5 openings for black, c5 as a gambit pawn, and why they seem to work for him no matter what his opponents play as White? He wins often also. I would be curious about books on it too. Thanks in advance.