It should’ve been crystal clear to her why she was put on it and why it’s important to take it.
From the OP it seems like the AFib is a bit of an afterthought, and not the main reason for her going on it that it should be.
A clot forming and floating around in her atria (top chambers of her heart) from stagnated blood (because they’re not beating right) combined with high blood pressure puts her at higher risk for a stroke or clot elsewhere in her body when the clot breaks loose and travels. This is not something she wants to have happen if it can be avoided. (Often hindsight is 20/20.)
This generation of blood thinners are easier than blood thinners of the past as
@seshat0120 has described above.
Lots of people are on these medications for many reasons.
If she has questions I wouldn’t wait to discuss with cardiologist, I would call the doctor‘s office who prescribed it to get any questions or misconceptions cleared up.
Substituting aspirin isn’t ok, it it was they would’ve prescribed aspirin.
She may not need to stay on it forever if they can get her out of AFib and back to a normal heart rhythm, but that’s going to be something she’ll talk to the cardiologist about. If it’s relatively new there’s a better chance of turning it around.