Continued . . .
Christ died once for all. His death was sufficient and accomplished its necessary and central role in salvation. However, the Body and Blood from that single and sufficient death is miraculously multiplied by Christ and has been made present on the countless altars of the world since the Resurrection. This Flesh and Blood from the one “bloody sacrifice” is offered to God again and again to propitiate the sins, which continue to be committed, to give thanks for the new moments, people, experiences, and events which comprise God’s ongoing creation, and to allow us who are alive today to fulfill the Lord’s command to eat His flesh and drink His blood: “He that eateth My flesh, and drinketh My blood, abideth in Me, and I in him.” (John 6:57.)
Thus there are four relevant meanings to the word “sacrifice:” This word is used to refer variously to
(1) the death of the victim;
(2) the victim itself;
(3) the priestly act of offering gifts to God; and
(4) the entire action of worship, including the dedication of the victim, its slaughter, and the offering of the flesh and blood on the altar.
If your friend will acknowledge these four meanings, you may be able to bring her to appreciate that:
(1) Christ died only once for all; this “sacrifice” is not repeated. (And she is right to hold that it should not be repeated.)
(2) Jesus Christ is the one and only worthy victim, the only fitting Paschal Sacrifice.
(3) The miraculously multiplied Body and Blood of Christ are “sacrificed” over and over again, in the sense that they are offered constantly and repeatedly (“re-presented”) to God as the only fitting worship of the Father, made possible only by the love and obedience of the Son. And, finally,
(4) The action of the “sacrifice,” which began with the prayers at the Last Supper and compassed the Lord’s death on Calvary, is the same “sacrifice,” the same single act of worship, that is continued on the altar of each Holy Mass; the perfect sacrifice of Calvary is thus perpetuated through all time.
When one keeps these ideas in mind, it is easy to see how each of the other posts in this thread, and the Church teachings on which they are based, are all correct and understandable, though they may not seem consistent until one has sorted out these four different uses of the word “sacrifice.”
I hope this is helpful and that you and your friend enjoy and profit from your discussions.
Spiritus Sapientiae nobiscum.