My daughter's friend called me "dad"

My daughter Lily (16F) has a friend named Kiera (16, turning 17 in two weeks). It's usually one of two situations: either Kiera is at our house, or Lily is at hers. They are very close. Kiera lives with her grandparents because, from what I understand, her dad abused her physically and sexually. It’s very sad.

One day, Kiera was at our house while I was making dinner. She walked up to me and asked, "Hey, Dad, what are you making?" I looked at her and said, "Did you just call me Dad?" Kiera started crying and seemed really sad. I think it was a sweet moment, but now I feel really bad about it.

Here is an update: I saw Kira in the bathroom doing her makeup. I woke up and said, "Hey, Kira, can we talk?" She said yes. I looked at her and said, "I'm sorry for the way I reacted. I was just super surprised. I love the fact that you called me Dad. You're kind of like a second daughter to me. I'm not offended, and Lily isn't offended either."

Kira said, "My dad used to beat me, so I never really had a good father. My grandfather is very emotionally distant. You're the closest thing to a father that I have."

I gave her a hug and a kiss on her head. Then I took my girls to get ice cream.