Ancestry Visa - Adopted - UK birth grandparent

Does anyone know if an applicant can be eligible for an Ancestry Visa on the basis of a birth parent born in the UK, where there has been an adoption?

My parent's is looking to apply for the Ancestry Visa. His mother was adopted and the birth mother (i.e. applicant's blood maternal grandmother) was born in England. Can my partner be eligible for an Ancestry Visa on that basis (provided the other criteria are met)?

The applicant's mother's Birth Certificate would detail her England born birth mother as her "mother" but also note the fact of adoption and her adoptive parents.

I would be very grateful for any help or insight. I haven't been able to find any clear answer to this in the Immigration Rules or Caseworker Guidance, or elsewhere online.