Like several other posters, I too, disagree with this claim. But rather than simply repeat our mantra (“Once a Jew, always a Jew.”), I’d like to go a step further and suggest how this error came about. Urban Legends are much more easily countered once you understand their sources.
Namely: We should distinguish between the obligations of a Jew, and the privleges of being a Jew.
A Jew has certain obligations - let’s take eating matzah at a Passover Seder as one simple example - and implicit in the concept of “obligation” is that it is not optional, one has to do it. One cannot escape that obligation simply by switching to another religion, any more than a US citizen might escape his income tax obligation by becoming a citizen of another country. (I’m talking about the tax on income which he earned while still in the US with US citizenship; once the obligation to pay tax is incurred, moving elsewhere won’t help.)
On the other hand, there are certain priveleges to being Jewish, such as counting towards a minyan, the minimum quorum of ten needed for certain synagogue rituals. Burial in a Jewish cemetary is another. Historically, many Jewish communities have withheld these priveleges from people in order to apply social pressure against defections to other religions. In olden times, when Jewish identity was very strong, and almost everyone observed the laws to the letter, one might lose these priveleges merely by publicly working on the Sabbath.
As time went on, such measures would have less and less, and would be used only for more serious breaches of one’s Jewish identity, such as marriage to a non-Jew. Nowadays, when so many people have such weak ties to the Jewish community, many feel that even people who have intermarried should not be written off in this manner. But through it all, it has always been considered reasonable that if a person has voluntarily left Judaism for another faith, then that person, by his own admission, should not be entitled to the priveleges of being Jewish.
Then again, despite such sanctions being imposed on such a person, he cannot simply “opt out” of the obligations of being Jewish either. The bottom line remains that he is a Jew, and if she is female, then her children will be Jews as well, as well as her daughters’ children, ad infinitum.
If any of these people chooses to rejoin the Jewish people, then technically no conversion of any kind is necessary. Practically speaking, however, the community and/or rabbi might request or require some kind of ritual, in order to impress upon the person the importance of this rejoining.
Also, in cases where the rejoining is after several generations – such as in Revtim’s case – it might be advisable to go through a conversion ceremony, just in case some misinformation got mixed up into the story. In other words, to whatever extent we are sure that Revtim’s maternal great-great grandmother was Jewish, then we are sure about Revtim as well, but to whatever extent we suspect that there might be some error in this newly discovered information after four generations, then CYA.