Is the pharmacist's testimony considered a false testimony and a sin, after she testified that the type the customer requested was what she gave her, and then it became clear to her later that the woman was right and that she had erred in her testimony?

Your friend's error in testimony is not false testimony, because she did not intentionally lie, but rather made a mistake, and this negates the ruling of false testimony. Nevertheless, she acted improperly and fell short in her duty when she testified merely because she remembered her friend. Her duty was to refrain from testifying if she was confused or did not precisely remember what happened; this is based on the Almighty's saying: "And when you speak, be just, even if it is against a relative," and the Prophet's (peace be upon him) saying: "Like this [meaning the sun], bear witness or refrain."