And this issue was up to the priest to deal with during the RCIA process.
Parish priests take this issue really seriously, which is why you have to fill in all the forms in RCIA.
And since he baptized this person he must have decided that the first baptism (if there was one) was not valid (most likely because it did not use the Trinitarian formula).
It is my understanding that she has never confessed this particular sin ever, to anyone. I was only thinking that at some point it would have been included in an initial talk with an RCIA priest or a subsequent priest. If I wrong, then I am wrong.
She did not and does not need to confess this particular sin, nor any other, mortal or venial committed before her baptism, because, as noted above, her priest baptised her on admission to the Catholic Church.
But if you are not careful you could also end up unsettling, upsetting, or unbalancing a person who has already been validly forgiven of all her sins, and who is trying to start a new life in Christ.
This ^ concerns me deeply.
I’ve never seen an RCIA questionnaire. What questions are asked? Do they ask about various sins of the past or just about your religious background? Thanks.
No

no way do the RCIA forms ask about your past sins

those would be dealt with exactly as they are for a cradle Catholic being confirmed, in the confessional, not on some form that anyone could read! But you do need to say if you have been baptised and into which church, then you need to produce the original, or certified copy of your baptism certificate so that the priest can make sure that the church that baptised you gave you a valid baptism.
On the lists I have found – Church of Christ is noted as valid or at least one to look into the specifics.
…and on looking into the particular Church of Christ, based on the certificate presented the priest baptised this lady, thereby washing away ALL her prior sins.
I would hope she knows she has been forgiven, but that does not mean that she does not still feel regret for her action, and particularly at this time, with abortion and the March for Life looming so huge - she does not need to be given a dose of guilt to add to whatever deep feelings she is experiencing.