A husband and wife started attending the Protestant Evangelical Church I attended for years before I reverted back the Catholicism. This married couple was born Catholic and married in the Catholic Church. Is it my place to tell them they should think long and hard (not in those words), before they give up The Eucharist as Catholics? I would feel very uncomfortable intruding.
Unless you know why they are leaving it is difficult at best to hit and miss with guesses.
If it has something to do with something about the catholic faith in faith or morals, you might offer them a copy of Scott Hahn’s book Rome sweet home.
If they were hurt in the way they were treated, then that is another issue.
And if their faith is just plumb tired out and boring, that is something else.
And then it might be something moral that is not acceptable to the catholic church but they want to do it.
It is a problem when you don’t know what is bugging them or even if they will tell you what is really troubling. There could be so many reasons.
I would say to offer them to Mary to take under her wing, and say a prayer for them every day to Mary if you can. “Immaculate Heart of Mary, pray for us now and at the hour of our death.”
It is good of you that you are interested in them and want to help.
May God bless and keep you. May God’s face shine on you. May God be kind to you and give you peace.