There is not a year 0 in the Gregorian calendar. It goes from 1 BCE to 1 CE. I understand that there is one in the astronomical calendar, but as I am not an astronomer I could not tell you anything more about it. What else do you need help with?

I am not a historian, but I do teach middle school math.

I think you'd be much better off comparing/contrasting a timeline that extends to BCE with a number line they see in math class. If you're in a state aligned with the Common Core, students do some in-depth exploration with number lines and extending them to the negatives starting in 6th grade so maybe your friend could coordinate with the math department (no pun intended).

Talk about how BCE is like the negative numbers while CE is like the positive numbers. Have a discussion about why there is no "year 0". For a more advanced discussion, you could also talk about how while the distance between every integer is the same, not every year on the Gregorian calendar is the same length (leap days).