Below may be where the catholic is getting this from.
I've often wondered why roman catholics are so insistent on this. I do not think they realize what they are saying and how they are being duped.
The salutation of the angel Gabriel -- chaire kecharitomene , Hail, full of grace ( Luke 1:28 ) indicates a unique abundance of grace, a supernatural, godlike state of soul, which finds its explanation only in the Immaculate Conception of Mary. www.catholicencyclopedia - immaculateconception
That is indeed part of the propaganda. Rather than only, that language applies to all believers. The word for “full” is not even in Lk. 1:28, as kecharitomene (one form of the verb "charitoo") in Lk. 1:28, is never used for "full" elsewhere, but Lk. 1:28 simply says she was graced, favored, enriched with grace, as in Eph.1:6. In contrast, the only one (though in some manuscripts Stephen in Acts 6:8) said to be full of grace is the Lord Jesus, "full ("plērēs) of grace (charis) and truth," using "plērēs," which denotes "full" 17 other places in the NT. If Mary was uniquely perfectly full of grace as bearing Christ then it would say she was, as Christ was, (plērēs charis) and RCs would not have to engage in such egregious extrapolations in seeking to left this invention.
Your own official RC Bible for America does not say “full of grace,” and Lk, 1:28 was wrongly rendered "full of grace" in the DRB, rather than "highly favored" or similar, as in Rome's current official New American Bible, “Hail, favored one!" (http://usccb.org/bible/luke/1) Yet the DRB correctly translates Eph. 1:6 as "in which he hath graced us."
For as CARM finds,In Greek: καὶ εἰσελθὼν πρὸς αὐτὴν εἶπεν Χαῖρε, κεχαριτωμένη, ὁ κύριος μετὰ σοῦ. κεχαριτωμένη, is the pf. pass. ptcp. of χαριτόω (charitoō). It is the single Greek word kexaritomena and means highly favored, make accepted, make graceful, etc. Repeated: It is a passive participle derived from charitoō. It does not mean "full of grace" or ‘completely filled with grace’ which is "plaras karitos" (plaras = full and karitos = Grace) in the Greek. More technical data from source here
Mary is said to be “full” of grace, or uniquely so, nor from what i find does kecharitomene being a perfect passive participle translate into meaning a "a perfection of grace," or distinctively a past action, as per RC argumentation, in distinction to echaritosen (another form of the verb "charitoo") used in Eph. 1:6, as there also it refers to a present state based upon a past action, "To the praise of the glory of his grace, wherein he hath made us accepted [echaritosen] in the beloved." (Ephesians 1:6)
See more on this issue here as White gets into detail with the Greek. (And notes that the fact that the Roman Catholic Church has to attempt to build such a complex theology on the form of a participle in a greeting should say a great deal in and of itself.)
Even Roman Catholic apologist Jimmy Akin said of Luke 1:28 on the word kecharitomene:
"This is a Greek term that you could use in that exact grammatical formation for someone else who wasn't immaculately conceived and the sentence would still make sense" like Mary's grandmother). He went on to say, "This is something where I said previously, we need the additional source of information from tradition and we need the guidance of the magisterium to be able to put these pieces together." Meaning the text does not teach the IM, nor is that necessary, but tradition becomes binding doctrine under the ultimate presumed authority of Rome.
Nor is “blessed art thou among women” a unique type of appellation, as Scripture also says, “Blessed above women shall Jael the wife of Heber the Kenite be, blessed shall she be above women in the tent.” (Judges 5:24)
It remains that while Mary is highly blessed among women, and is to be honored according to what is written, this does not translate in the type of supererogation of praise seen in Catholicism, in which humble Mary is made into an almost almighty demigoddess!