I wouldn’t mind, honestly. I’m an easy come, easy go type of person.
BUT , a member of my family was very annoyed with me, when I took my AGS with me. This family member, had invited me for lunch. My AGS happened to pop in to see me just as I was leaving. So I said, i was going over to N’s house. He loves his cousins, so he wanted to go with me. I didn’t think there would be a problem. In my house, I am a generous cook, and there’s always more than enough to feed an extra two.
I have learned my lesson. Not everyone is like me.
In your case, OP, it’s different. You were going out for a meal. Thats an extra £20 -£25.
I have a feeling, the guest will pay her share, hopefully. Enjoy whatever is decided !