My father actually collects fountain pen, he got published many times in fountain pen related magazines and he is no doubt in the top 100 collectors in the world, and top 1-5 for specific brands.
He never has expressed anything about the quality of paper. Actually, he takes his notes on whatever is available, which often means the back of an envelope lying around and otherwise printer paper. I have never seen him use a notebook actually, let alone an expensive one.
You're very lucky. I wish I had a chance to chat with your father. I also collect fountain pens, but not with that laser focus.
He probably has a favorite ink to use with his fountain pens, since inks change both a pen's behavior and their interaction changes from paper to paper.
There's a list of popular, run of the mill inks which work very well with lower quality paper. I also use these inks with my daily driver pens and notebooks, since they're easy to maintain, looks nice, and works with almost any notebook out there.
However I had some notebooks which were not writable unless you use a gel, ballpoint or some rollerballs. Fountain pen ink just stayed as little droplets and never dried on them.
He never has expressed anything about the quality of paper. Actually, he takes his notes on whatever is available, which often means the back of an envelope lying around and otherwise printer paper. I have never seen him use a notebook actually, let alone an expensive one.