It would actually be illogical for me to demand the atheist agree with my conclusion, unless and until he came to agree with my proposition about the truth of the Bible. This was before the Fall and rebellion of humanity. Chapter 1 of any biology textbook gives the definition of life, and according to that definition plants are alive. The logic here is perfectly sound. However, there is a straightforward answer from a Christian perspective.
Life is defined in terms of having a spirit, the nephesh.
The spirit, in turn, is closely associated with blood. Higher animals are alive because they have blood, but not plants or microbes. Thus, there was no death associated with the vegetarian diet of the original creation. Most Christians trained as secular biologists probably are not aware of this distinction in the definition of life, and would misunderstand the Bible for this reason. In order to convince your opponent your position is right, you will first have to convince him there is a problem with his own position.