2nd marriage fixed but did received divorce decree for 1st marriage

My sister had been separated from her husband for a year after that both decided to dissolve it through divorce by mutual consent. After filing the petitions and following all the legal proceedings for another year, she got fixed for a marriage after the said date of handing over the decree of divorce. But somehow it got skipped. So the next date was agreed upon. But then her ex got to know and filed an application to skip the date, which now falls after remarriage date to seek revenge. How to counter this without stalling the marriage