Q:

If a convert goes off the derekh, must he go to the mikvah again after he repents?

A female convert I know, who had stopped observing mitzvos for a few years, but then sought to return to the fold, was led to believe by someone that she would have to go again to the mikvah to confirm her Jewishness. I had never heard that before and now I have reasons for being suspicious of the advice, given its source.  Is this halacha?

A:

Shulchan Arukh, Yoreh Deah 268:12

‏...ואפילו חזר ועבד עבודת כוכבים – הרי הוא כישראל מומר שקידושיו קידושין.‏
ישראל מומר שעשה תשובה – אינו צריך לטבול. רק מדרבנן יש לו לטבול ולקבל עליו דברי חבירות בפני שלושה
...even if the convert returned to worshiping idols -- he is still a Jew such that his marriage proposals are binding.
An apostate Jew who repents -- he does not need to dip [in a Mikva]. Only rabbinicly he should dip [in a Mikva] and accept upon himself words of companionship (?) in front of three [people]. (my translation)

So a custom like that is on the books. But it doesn't mean they aren't Jewish in the meantime. It's a sort of formal "welcome back" ceremony.
You should have her speak to a rabbi to see how her situation compares to this one.

