It’s easy to say “No”. It’s also easy to be an ass and make enemies.
What’s hard is saying “No” in a way that isn’t rude, and doesn’t make enemies.
You may not think you need to make the effort to be nice when rejecting people that ask you for stuff.
You may not think it matters whether you have enemies.
But why take the chance?
Maybe that person crosses your path in ten years and you need something from her.
Or maybe karma is a real thing.
Either way, learn to say “No” nicely.
There’s zero downside.