Is it permissible to fast for two months as an expiation for a sin, if I advised a pregnant woman to drink herbs to cleanse the womb, and she did not know whether the soul had been breathed into the fetus or not, then she miscarried because of this?

You are not obligated to do anything regarding what happened, and there is no need for your apprehension, because your friend did not bring what you advised her to, and the hospital had decided on the curettage operation. The ensoulment occurs after four months, and the fetus was two and a half months old. If a cause and a direct perpetrator are present, then the liability falls on the direct perpetrator. The blood money (diyah) and expiation (kaffarah) are due for aborting the fetus after ensoulment or after its formation. Furthermore, the retention of a dead fetus in the mother's womb is dangerous for her, and aborting it with herbs is better than a curettage operation.