David Lewin wrote: I am in correspondence with a woman born 1953, who
suspects that her mother did not give birth to her, and is asking for help
to find her birth parents.
David - as you rightly point out, in the first instance your correspondent
needs to clinically prove that she is not her registered mother's daughter.
The simple process would be for both parties to have an mtDNA test - and if
they were significantly different then that would prove the first part of
her case. However, clearly this is not a simple question of each party
agreeing to and then having the appropriate definitive test!
Just to add one comment.
Ideally the woman would test with her registered mother to find out if
they are biological mother and daughter. However, if that's not possible
due to a strained relationship, an autosomal test with any other close
relative - a sibling, aunt or uncle, or a first cousin, may be sufficient.
It will tell her with a good degree of confidence whether she is
biologically part of the family. Hopefully there is at least one such
relative who is willing to help.