Who was at fault?

I entered into a parking spot, but needed to readjust a bit and so put on reverse and backed a bit until I heard (did not see the car even though I was checking behind) a bump noise and I immediately hit the break and got out of the car.

There was another car and the lady said she was getting out of her parking spot in reverse and also did not see my car.

We checked and there was no visible damage; we probably stopped our respective cars immediately and were going in extremely low speed and that helped. We thought of exchanging info in any case, if in the future sth appears, but her husband and my bf said there was no damage, so there was no need. My bf says she was at fault, but it doesn't matter.

Who was at fault? What do you think?