There is a rabbinic decree prohibiting single women, including divorcees, from immersing in the mikveh. However, there are varying opinions as to whether the decree applies to immersion for the sake of teshuvah prior to Yom Kippur.
It is usually up to a local mikveh’s rabbi to determine the mikveh’s stance on this issue. You may wish to discuss it further with the woman you consulted. You could direct her to the responsum written by our site’s rabbinic supervisor, Rav Yehuda Henkin. He permits single women’s immersion for teshuva on Erev Yom Kippur, subject to community custom and local halachic policy. It is available in the original Hebrew
here, and in English translation in his book
Responsa on Contemporary Jewish Women’s Issues.
Gemar chatimah tovah!