Hi there! My name is XXXXX XXXXX I will be here to help answer your question.
You have a good question, and I understand that installing the granite first seems counter-intuitive, but I would tend to agree with your contractor friend. There are a few reasons to install granite first:
1. Granite is heavy. It will push down on the wall you have built ever so slightly. You want the wall and framing to settle first before you add a finished product to it, in this case tight-joint slate tiles.
2. If one of your tiles sits slightly higher than the framing, the granite would end up resting on the tile and not the framing, likely causing it to crack and/or pop off.
3. Might not be as important, but the granite would then give you a straight line to measure your cuts for the slate.
Building is not always a bottom to top process.
Hope this helps. Let me know if you have any further questions!