Strawman Fallacy

The other day I was arguing with a Christian guy on God and we were challenging each other in random religious matters. So in the course of debate I asked him if he could prove god's existence. He, instead of answering, counter-challenged me if I could prove that Jesus was decayed in that tomb. I was startled by his ignorance and told him that he couldn't counter-challenge me without answering my question and also added that I did't regard Bible as valid research.

So here I have suspected that he had made Strawman fallacy.

That wouldn't be a strawman, but there are a few things going on here. First, asking to prove that Jesus was decayed tomb is actually more reasonable than proving that God exists, so perhaps your friend's retort was just to prove a point (i.e., the kind of proof you are asking for is not possible on either side of the debate). Your friend is shifting the burden of proof by asking you to prove Jesus was decayed in a tomb. If he is claiming that God exists, then he needs to produce evidence for the claim or admit that he just believes on faith (and revise his claim from of claim of existence to a claim of belief).