My friend passed away. She was unmarried, had no children, and no siblings. Her biological parents are still alive, and she had been adopted (simple adoption). Do the adoptive parents inherit?

Yes. Article 368-1 of the French Civil Code provides that the estate of a person who was adopted under a simple adoption is divided into two parts: one half goes to the biological family, and the other half to the adoptive family (Article 738-1 of the Civil Code).