We have never heard of the custom mentioned by the kallah. It is fully permissible for her to immerse even if she expects her period within the next few days.
It is often medically possible to delay a period, and we recommend that she speak with her doctor about this as soon as possible. If her period is regular, and she expects it on her wedding day,
progesterone pills can be taken beginning about five days before she expects her period and continued until after they consummate the marriage. This would delay her period for the few days necessary and, unlike birth control pills, these are taken for just a short period of time. Note that these are
not the same progesterone pills normally prescribed for contraception.
Please feel free to get back to us with any further questions.
B’hatzlacha!