marriage - Why does someone wanting to get married need to show an Orthodox ketubah from a maternal ancestor?

My friend is due to get married soon and has been asked by her Orthodox Rabbi to provide proof that she is jewish by showing that she is descended from jews. She doesn't have her parents Ketubah so the Rabbi said that she could provide evidence that her maternal grandparents had a jewish wedding as evidence instead.

She has looked and her grandparents seem to have been married by a Conservative Rabbi. Her Rabbi said that this wasn't good enough as the wedding had to be Orthodox to be accepted. Apparently this is also the view of the central beth din. This means she can't have an Orthodox wedding according to at least one country's rules of Orthodox judaism.

What I don't understand is that Conservative, Reform, Orthodox etc. judaism didn't exist as a concept before the 19th century so presumably if you look far back enough, all jews are descended from the same groups of people. And it seems clear that if my friend could establish that her maternal great-grandmother, say, was Orthodox that would do no matter what her grandparents practised. So what is the distinction that today's Orthodox jews are trying to make? It doesn't make sense to me.

Are Orthodox jews worried that someone in her maternal ancestry might have converted to Conservative Judaism (see e.g. http://judaism.about.com/od/conversion/f/consconv_accept.htm) and so she would not count as a jew at all in their eyes if that had happened? Or are they worried that non-Orthodox Rabbis would allow intermarriage (see e.g. http://forward.com/articles/142112/conservative-synagogues-crack-open-door-to-interma/) and so the wife might not have been jewish at all?

Answer

A Conservative kesuba can be suspect. As noted in Rabbi Emanuel Feldman's book, "Tales Out of Shul," as a young Orthodox rabbi in Atlanta in the 1960s, he was under tremendous pressure by wealthy members to do a quicky conversion to the member's child's non-Jewish fiancee. He would refuse, and those members would leave the shul and join a Conservative shul, making large donations, and getting what they wanted. The result was that Jews were married to non-Jews who were not sincere converts. That is reason for great concern.