Only the Scriptures can prove right and wrong. Christian tradition informs us how the earliest Christians interpreted Scripture.
Many Catholics and Orthodox will contend that praying to saints for them to intercede on our behalf, such as Mary, is a practice very well grounded in church tradition. I do not think this is the case nor do think it makes sense with the Scripture either.
Let me quote at some length Augustine’s account of his mother in Book VI of the Confessions:
When, therefore, my mother had at one time— as was her custom in Africa— brought to the oratories built in the memory of the saints certain cakes, and bread, and wine, and was forbidden by the door-keeper, so soon as she learned that it was the bishop who had forbidden it, she so piously and obediently acceded to it, that I myself marvelled how readily she could bring herself to accuse her own custom, rather than question his prohibition. … [B]ecause these, so to say, festivals in honour of the dead were very like the superstition of the Gentiles, she most willingly abstained from it. And in lieu of a basket filled with fruits of the earth, she had learned to bring to the oratories of the martyrs a heart full of more purified petitions, and to give all that she could to the poor; that so the communion of the Lord’s body might be rightly celebrated there, where, after the example of His passion, the martyrs had been sacrificed and crowned.
So, what this appears to me is a description of Monica dumping an old African practice of essentially partying with the dead and bringing food for them along with other worshipers partying with the dead martyrs and replacing it with a solemn worship service where prayers are offered and the sacrament of the Lord’s Supper is practiced.
Now, I think if you wanted to, you can legitimately can interpret the text to mean that Monica once offered the Saints food but now offered them prayers to themselves. However, I will tell you from my reading of Augustine, why I think that is not the case.
In reading Augustine, you will run into tons of theology interrupted by prayers to God, then back to theology, and then back to prayers.
If Monica was indeed praying to Saints instead of praying to God and partaking in a worship service at the martyr’s tombs, why does Augustine never offer a single prayer to a saint in any of his writings asking for intercession? This indeed is an argument from silence, but it also takes an argument from silence to say Monica was praying to saints rather than to God at the tombs after she was corrected by Ambrose. However, we do have a litany of examples where Augustine prays specifically to God. The Scripture has only these as well.
Further, how is Monica offering prayers in place of food to the dead? The more logical conclusion is that instead of offering the dead martyrs food she offered them prayers to God on their behalf (which is actually the opposite of praying to a saint so they may pray on our own behalf.) If she was asking the saints to pray for her, this would hardly be an offering to them like the food used to be.
Now, I would have questions about this practice too (and if you read this link, it is this sort of prayer that Hippolytus was offering) but the object of prayer is God and not the saint, so I would say it is as permissible as praying that someone you loved rests in peace or that God may have mercy on their immortal soul.
It is for this reason, we should follow this example and not the example of anyone else who asked for intercession in the name of someone other than God. Because the Scripture is clear, no one intercedes for us other than God alone:
In the same way the Spirit also helps our weakness; for we do not know how to pray as we should, but the Spirit Himself intercedes for us with groanings too deep for words; and He who searches the hearts knows what the mind of the Spirit is, because He intercedes for the saints according to the will of God. (Rom 8:26-27)
Christ Jesus is He who died, yes, rather who was raised, who is at the right hand of God, who also intercedes for us (Rom 8:34).
For there is one God, and one mediator also between God and men, the man Christ Jesus (1 Tim 2:5).