There's a proper proof, which is too long to fit in this margin (i.e. I'm too lazy to find a link), but I can do a much shorter one:

1. Non-determinism is equivalent to determinism plus extra, random input.

2. The addition of random noise does not bestow free will upon something that didn't have it without the random noise.

3. 1 and 2 imply no non-determinstic systems can possess free will if deterministic systems cannot possess free will.

4. 3 implies free will is independent of determinism.