If this is true, then out of deference to the poster, you should provide the Canon Law text to support it. From what I understand, there are only 6 or 7 areas where excommunication still applies, and even those can be subject to defense.
If Catholics attempt to marry before a non-Catholic minister, they not only commit sin, but they are excommunicated from the Church. They are not married (C_of_D note - I was wrong on this last point).
They are excluded from the sacraments, may not be godparents for baptism and confirmation and may not receive Christian burial. Their excommunication lasts until they go to confession, receive absolution from the bishop, and get married before a Catholic priest, if they are to live as spouses.
NYer, neither you nor I are priests, theologians or canon lawyers. The best advice is to send a person to a qualified priest. Laypersons have no business giving reassurance or condemnation on this matter. It's complicated.
A family member of mine was married outside the Church in the 60s. She was excommunicated. She could not get her baby baptized in the Catholic Church because of it. She and her husband were remarried in the Church and the baby was conditionally rebaptized.
The rules may have relaxed since VII or maybe not. The local N.O. parish advises those not married in the Church to refrain from Holy Communion and make an appointment with the pastor.