Can a woman use a Mikvah if her Husband is not Jewish (She is currently on the road to becoming a BT)
Tough question. I know a lot of nerves can be frazzled by this one.
In Temple times, when ritual purity affected all sorts of aspects of daily life, this might be a different question; but today, women's regular use of mikva is only to end the status of nida.
On the one hand, the woman would like to increase her religious observance (a wonderful thing), and this is certainly a major act.
On the other hand, from a strict Halachic perspective, she's accomplishing nothing:
ד,ד הגויים--אין חייבין עליהם משום נידה, ולא משום זבה, ולא משום יולדת. וחכמים גזרו על כל הגויים, זכרים ונקבות, שיהו כזבים תמיד, בין ראו בין לא ראו--לעניין טומאה וטהרה.
Rambam Hilchos Issurei Biah 4:4
To paraphrase: the laws of Nidah only pertain to a couple where both is Jewish. If either partner is non-Jewish, their nida status (and hence mikva observance) makes no difference.
So it's something that would make her feel better about herself, but is (frustratingly) of no Halachic significance. This is further complicated because in many cases, mikva use is really limited to those who Halachically need it. (See for instance this question).
ובאמת בנידון דידן אין קידושין תופסין ואולי אסורה לטבול כשאר פנויות, אבל אין כך עונין המעיקות
You can try asking yoatzot.org if you like. The final decision may come down to the rabbi or women in charge of the local mikva, but those are the issues that I know of.
There is no halakhic reason why this woman cannot immerse in the mikveh. She should be encouraged to make whatever decision is right for her.
The link listed in the previous response does not mention anything about immersing for non-required reasons - unclear what the responder's basis is for this. People have immersed for non-commanded reasons for hundreds (if not thousands) of years: before Shabbat, before Yom Kippur, 9th month of pregnancy.
A mikveh is "lo mikabel tumah" - it cannot receive impurity. As long as it was built under rabbinic supervision and is maintained under rabbinic supervision, there is no way it can be "messed up" for anyone else because of who goes in, when, and for what reason.