We appreciate the sensitive nature of this question and will try to address your friend’s concerns.
In order for a bride to immerse and exit the niddah status, she must count seven clean days after the cessation of bleeding. Since your friend will not have enough time to check herself and count properly (details here), she should wait to immerse until after her wedding. We would encourage her to do so; this will still be significant and meaningful, marking her first immersion as a new bride. She can make a special appointment for after the wedding. This scenario is not uncommon, happening whenever a bride’s period concludes less than seven days before her wedding.
Even if a bride must wait until after her wedding to immerse, the wedding and marriage remain valid. The ceremony will still be holy and blessed, and the celebration will still be joyful and memorable.
As to your friend’s concern about the status of the mikveh: a woman who immerses without counting the full seven days does not render the water ritually impure. The water of the mikveh retains all its ritual properties for the women who immerse after her. However, as noted, she herself has not completed the taharah process properly if she did not wait to count seven clean days before her immersion.
We hope that you will be able to provide support and guidance for your friend at this milestone. May Hashem grant the new couple many happy years together.
Please do not hesitate to reach out to us with any further questions.
Updated 3 August, 2023