Fred is from Harry Potter, which just about everyone and their mother has read. I assume fourth-walling is sort of inevitable.
Yes, you can feel free to fourth wall Fred. I would prefer no outright "LOL YOU DON'T EXISTS" but Fred's a pretty resiliant guy, he can take it if you feel it's absolutely necessary. Go to town as far as telling him magic isn't real, wizards don't exist, etc, etc.
In the case of spoilers, I would prefer a bit of tact in that arena. Fred's pulled from literally minutes before his death, and I'd prefer if characters he didn't know don't fling that in his face since it will most likely end poorly for everyone involved.
Any questions, please leave them here.