What Is Lying?

So I was having a conversation with Mr. Olabowale and this is what he told me.

He said: "In the Quran if you are about to die and they ask you if you are a muslims, and you are afraid of death, then yu can tell them that you are not a muslim"

I told him that was lying and asked "doesn't the Quran consider lying a sin?"

He said that lying is a sin, but his statement above is not lying.
I asked him to give me a definiton of lying, he kept beating around the bush with it, so I still didn't get an answer.

I told him that the untruth or whatever is not the truth or saying that you are not something that you are is lying. I told him even a 3 year old would tell you that when you don't tell the truth you are lying.

He said that because I am young and only speaking from my experiences in life that I lack understanding. But I want to know what is the misunderstanding in "if you don't tell the truth, you are lying"

But anyway. I thought I was going crazy or something. So I won't ask a religious person this, I will ask an atheist this: What is the definition of lying, and would the above statement by Mr. Olabowale be 'lying'?

I would appreciate an answer from an atheist since they are quite neutral about it. Thanks.