Fulfilling a promise to God?

My mom (53) has breast cancer. We started this journey in November, when she went to the doctor for another thing and he noticed a weird lump in her chest wall and sent her to check it out. Official diagnosis came January 15th, and treatment (neoadjuvant chemo) will finally, FINALLY start next week. Treatment is supposed to last around a year total.

My mom is a very religious person. Not conservative, not in a Bible thumping way, but she does believe in God with her whole heart and has been praying a lot more since this all started. And though I'm not religious, whatever helps, right?

When she had skin cancer some ten years ago (very treatable, nothing compared to this) she made a promise to God that if it all went well we would both have to go to church every week for a month. I didn't LOVE this, considering I don't believe in God and I was a teenager back then, but I did go because it's my mom.

Now, I'm going to every appointment, making sure she does everything right, asking the questions she doesn't know to ask, doing all tangible things I can do. And then she asked me if I had made a promise to God if her treatment works out. I said no. Then she says "I did one, for both of us". And I was like, "What?" And she said "You'll know when the time is right."

Look. I'm a materialistic sort of person in the sense that I believe in what I can see, and the promise I made to myself and to the universe was to do be her rock through all of this. And I support her faith. I support anything that brings her strength. If it's something along the lines of going to church, fine. But I'm kinda worried she made some crazy promise that I'll not be able to keep for a God I don't believe in. Not sure how I'll say no in case it's something I'm not willing to do, since I know she takes this seriously.

The mystery only makes it worse. She should've at least either told me already or dropped it with whatever the promise is "when the time is right".