As for the first few sentences: There are lots of things to say. How about going to Jeremiah 31:29-30. Is Jeremiah 31 about the new covenant? If that's so, then how do you explain there are transgressors among God's people who are "eating the sour grapes" and having "their teeth set on edge" in the new covenant administration?
Those who make these assertions seem to not be able to see the biblical juxtaposition of outward membership versus inward membership in the covenant. This didn't just start happening in the NC. This was always there. Take a look at Esau. Was he a member of the OC? He started that way, then he took his belongings and his family, and left Canaan and God's people who dwelt there. He was a member of the OC outwardly; at least until he left. It was at that time he showed himself to have never been inwardly. Thing is, there's always been a difference between being IN the covenant and being OF the covenant. Not all in the OC who partook of the RITE (IE, circumcision) had the REALITY. And guess what. It's the same in the NC.
As for the question about girls being baptized, it's simply that in the OC the women were represented/included in the circumcision of the men (IE, the girls' fathers). Thomas Blake touches on this briefly with these words:
“For the exception of women, though foederate, yet were not to be circumcised, I say. . .that they were of the circumcision, and it was an exception against Sampson by his parents that he would go to take a wife of the uncircumcised Philistines (Judges 14:3). Had he married in Israel, as he ought in obedience to God and his parents, he had married a wife of the circumcised, though that sex by nature is in an incapacity of that sign or seal.” (Blake, pp435-36).
And again: “There are many things of which we make no question, and yet we have no example of them. . .for women's receiving of the Lord's Supper, there is not a particular institution, or any particular express precedent for it in the New Testament. . .One goes about to give instance of particular precedents for women's receiving the Lord's Supper, and instead of a precedent urges 1 Corinthians 11:28 as an express command in formal terms for women,
anthropos comprehending both sexes. . .We are then furnished with an express command in terms of formal, and with an example to boot of women's circumcision; and so the difference between circumcision and baptism (so often laid in the dish of paedobaptists) here falls to the ground; [for our Lord said]: 'Ye on the Sabbath-day do circumcise,
anthropos, a man (John 7:22). [And again] 'If anthropos, a man, on the Sabbath-day receive circumcision, that the Law of Moses should not be broken. . .' (John 7:23). Here is Moses' command; the Jews practice, with Christ's approbation, in the same comprehensive latitude in regard of both sexes, as in Saint Paul for receiving of the sacrament of the Lord's Supper. . .[So then, of the Lord's Supper in 1 Corinthians 11:28, it] is replied that the subject matter of the command as well as the grammar use of the word, proves females to be included. . .[But] if arguments borrowed from grammar use of words, be of that force; you see what they have proved: As
anthropos is taken in one sacrament, so it is to be taken in another.” (Blake, pp414-15).
Having said all of that, I don't think just answering the questions at face value will necessarily help. You have to first discern if this person is genuinely open to considering the truth of the other opinion. If so I would give them a solid book to have them go through that goes to the heart of the issue. Then have a conversation about it. If you're looking for a resource, this may help:
https://f5b3affa-3815-4a9f-8ecc-bd3...d/be37d2_2dc54c814e994ab4874e0ac1766ebea1.pdf