If the prior marriage was a valid, sacramental marriage, then only the death of one of the spouses frees the other to marry again. The Catholic Church is consistent with Jesus’ teaching on this (see Mark 10:9).
Therefore, if one of the spouses wishes to remarry in the Catholic Church while the other spouse is still living, the prior marriage must first be investigated to determine whether it was valid and sacramental. If it was, a new marriage cannot take place. If it was not, the Church will declare this officially with a declaration of nullity and the party should be free to marry a Catholic in the Church if the Catholic receives the appropriate dispensation to marry a non-Catholic.