There seems to be such a basic, fundamental lack of understanding of the scientific process, and of the very purpose of science, by the religious-minded.
As Simon17 already pointed out, the way science works, and the reason it works so well, is because it is tremendously easier to DISprove a theory than to prove it. A good scientific theory will include predictions based on the theory's assumptions. If it can be demonstrated even in one instance that the theory does not hold true, then the theory is false. This, however, is not a failure of science but a success. It is just as important to determine what is false as to determine what is true, because not only do we want to believe true things, but we want to NOT believe FALSE things.
The concept of "belief" is very dangerous because we will always be biased toward supporting our beliefs even in the face of contrary evidence. The Watchtower says you MUST believe something; if you don't believe them, they ask, "Where else will you go?" This, however, is illogical. It is entirely possible that you simply don't know where to go, but that does not mean you should stick with the Watchtower even though they give every evidence of being wrong.
In my opinion, the willingness to admit 'I don't know' is the start of learning and a requirement for anyone seeking truth. When your starting premise is a belief instead of an awareness of your own ignorance, you cannot but create a fantasy world of beliefs rather than perform an honest evaluation of evidence.
So asking someone to prove a scientific theory may be impossible, depending on the asker's definition of proof. The fact that reliance on scientific theories will let us land men on the moon may "prove" certain theories about physics to some people's satisfaction, since presumably, if the theories were false, reliance on them for such a delicate mission would only end in disaster. But a reasonable person could suggest that the theory (for example, of gravity) is currently incorrect in some way, positing that while the current model does allow space missions to succeed, the theory breaks down under certain not-yet-tested conditions. Thus, to such a person, the theory of gravity, in its current form, is yet unproven.
So when xjw says "prove [macro]evolution," we really don't know what form of proof would be acceptable, since she has declined to state any proof conditions. I suspect that she has no formal college education; certainly not in any scientific field, or she would understand the vague and invalid nature of her request. It's similar to asking her to "prove that The Watchtower is not God's approved channel."