I think your friend doesn't know much about how hard drives or Windows functions. What your friend said used to be true back when people were using HDD's with much smaller capacities. They were using like 99% of their tiny hard drives and not leaving enough room for Window's paging file. As long as you leave at least 16 GB free (Windows shouldn't ever need more than this to function normally) then you'll be fine. You will not notice any significant difference in performance if you use more than 50% of your 1 TB HDD.
Precisely. Never filled a hard drive with Windows but Linux does it OK as long as you have enough room for OS operation (virtual memory, logs, etc.).
It's like the old rule of thumb of having a swap partition twice the size of your RAM. Nonsense nowadays. At the most, just the size of your RAM if you run a laptop that must suspend and dump RAM to disk. I normally use 2 GB just because.