I know it means "one friend", literally.

My context is this. In a restaurent where English was mostly being spoken, I said "Tes,ekkürler" to a Turkish Waitress who brought me something, she replied "Bir Dost", and some things I couldn't understand.

My take was she wanted to give a "friendly" compared to "formal". Is this correct?

In that context, I think, waitress might mean that ' this present is from a friend.' (Someone might like to offer you a present for courtesy without giving his own name)

Your answer feels right and adds to my understanding.

çok teşekkürler

Bir şey değil.