He replied that: "You don't waste time proving a negative."
This response is triply inadequate. Firstly, it is possible to prove negatives--every positive claim has an equivalent negative claim. What is difficult (though not necessarily impossible) to prove is a negative existence claim or a universal negative. Secondly, "proof" is best left in the realm of logic and mathematics. In science what counts is the weight of evidence. Thirdly and most important: the claim that the "candle effect" is the explanation of SHC is not a negative claim!
Skeptics need to realise that just because an explanation is consistent with established science doesn't mean it is necessarily correct--evidence is still required to support it.