Let's translate the Greek LXX text as literally as possible (slavishly), and then translate it properly into English. And we'll then look at the relevant words to the question: out of what is man created according to this passage.
2 Maccabees 7:28 LXX
ἀξιῶ σε τέκνον ἀναβλέψαντα εἰς τὸν οὐρανὸν καὶ τὴν γῆν καὶ τὰ ἐν αὐτοῖς πάντα ἰδόντα γνῶναι ὅτι οὐκ ἐξ ὄντων ἐποίησεν αὐτὰ ὁ θεός καὶ τὸ τῶν ἀνθρώπων γένος οὕτω γίνεται
My Literal/Slavish Translation
I-beseech thee, child, look unto the heavens and the earth and all-that-is in them. See, understanding that not out of existing-things did God make them; and the coming-into-being of the human(s) race likewise.
I beseech thee, my son, look upon the heavens and the earth, and all that is in them. Understand that God made them out of no existing thing; and that so mankind was brought into being.
Vulgate Translation (Douay-Rheims)
I beseech thee, my son, look upon heaven and earth, and all that is in them: and consider that God made them out of nothing [Vulg: ex níhilo], and mankind also.
"From What is Mankind Created?"
In short, "out of nothing existing."
The relevant parts are οὐκ ἐξ ὄντων and καὶ τὸ τῶν ἀνθρώπων γένος οὕτω γίνεται.
οὐκ - ἐξ- ὄντων
NOT - OUT-OF - EXISTING THINGS
καὶ τὸ τῶν ἀνθρώπων γένος οὕτω γίνεται
AND - THE - HUMAN(S) - RACE - IN-THE-SAME-MANNER-[ALSO] - WAS-BROUGHT/CAME-INTO-BEING
That is, the 'made was thus also (οὕτω) made' answers to 'out of no existing thing' (οὐκ ἐξ ὄντων)—God didn't make man out of anything He hadn't created ex nihilo, God is creator of all things without exception.
The word ὄντων in ἐξ ὄντων (out of existing things) is a pluran noun form of the verb to be (ὢν —being, existing). As in (ἐγώ εἰμι ὁ ὢν) (I am the [simply] being [one]). And so means 'things in existance.'
Thus, οὐκ ἐξ ὄντων (not out of existing things, or, not of things that were) is simply exactly synonymous with "not out of anything," or, "out of nothing," as the Vulgate does render it. (cf. Romans 4:17b).
Some understandably consider this, on the surface, to be a contradiction of Genesis 2:7:
And the Lord God formed man of the dust of the earth: and breathed
into his nostrils the breath of life, and man became a living being/soul.
But this reduces man to his fleshly/bodily component alone, which is not a man, strictly speaking. And even then, this body was ultimately created out of nothing, which is the argument of 2 Maccabees: God is Lord of all creation, and did not use anything to make it which He Himself did not make. (John 1:3; Colossians 1:16-17). The Jewish mother was not ignorant of Genesis 2:7!
Whereas Scripture uses 'man' in this rare, singular, early, introductory usage and sense, to mean the body of the man, never before seen in Scripture, or in the narrative, on earth. That is, 'Man' here refers to the later, completed man, Adam. (Adam itself in Hebrew comes from a Hebrew word for earth/ground (אֲדָמָה 'adamah), and his name (אָדָם Adam) could be loosely translated '[made] of earth.' Since it's thought adamah is a feminine word, it could also be loosely translated 'son of the earth').
Where were we.
Man is definitionally a body-soul composite. That's why the spirits of the dead who are in heaven but not yet resurrected bodily are not called 'righteous men' but "the spirits of righteous men made perfect." (Hebrews 12:23)
Adam only became truly a man or human whenever he "being a living being." That is, He became more than a corpse. A corpse is not, properly speaking, a man.
So the specially created soul, infused into the body (Ecclesiastes 12:7; Genesis 1:27), becomes a man. In this sense, God didn't use anything to create man. In the sense that he 'clothed' the specially created soul with flesh, He took of creation: but He still created that out of nothing. I think this is the sense in which 2 Maccabees says 'God created everything out of nothing, even man.' That is, God made all things without exception, out of nothing. God alone is eternal.
2 Maccabees is saying that the same can be said of man's creation: he was not created of things which God did not create, but, utlimately, from His ex nihilo creation: and just as much specially, as to his immortal soul!
There is no way to translate or understand "ὅτι οὐκ ἐξ ὄντων ἐποίησεν αὐτὰ ὁ θεός" (that God did not create them out of anythingin existence) as creatio ex materia. It's the exact opposite that's being argued.
And that the Jews adopted some Greco-Roman belief as their own is not plausible at all. First, this view put forth in this passage is a reminder, used to strengthen the martyrs as they are brutally tortured in front of their own mother. It is not a suggestion of a new doctrine, but an already accepted doctrine of the Jews clearly already well established and believed (2 Maccabees being written in around 125 BC—predating Philo by about a century). Second, the books of Maccabees are about the Maccabean revolt against the Greeks for forcing upon them worship of the false Greek gods, being forbidden to keep the Jewish laws, and other apostasies and persecutions. They weren't disposed to readily accept what would be an alien doctrine like this (the exact opposite of any prior ex materia creation belief). They already believed that God created all things whatsoever, from nothing (cf. Colossians 1:16-17).