If any person unconsciously consumed non-vegetarian food and later learned that he/she has done so, is there any provision in Hindu religion to wash that sin?
First of all, in general, consumption of non-veg food items, by all people, under all circumstances is not a sin that scriptures will prescribe atonement measures for the act.
As clearly said in the following verse, giving up meat eating is greatly rewarding and is recommended too, but eating meat is "not that bad".
Manu Smriti 5.56. There is no sin in eating meat, in (drinking) spirituous liquor, and in carnal intercourse, for that is the natural way of created beings, but abstention brings great rewards.
However, this should not be understood as scriptures allowing unrestricted consumption of meat. Generally speaking, only sacrificial meat (i.e meat which is already offered to the Manes or the Gods first) is allowed (or recommended) to be eaten.
Manu Smriti 5.32. He who eats meat, when he honours the gods and manes, commits no sin, whether he has bought it, or himself has killed (the animal), or has received it as a present from others.
Even a Brahmin can eat meat, if he wishes, but only after sanctifying it with water infused with Vedic mantras.
Manu Smriti 5.36. A Brahmana must never eat (the flesh of animals unhallowed by Mantras; but, obedient to the primeval law, he may eat it, consecrated with Vedic texts.
Also, one can not refuse meat either if its offered to one on a sacred occasion (like say Shraddha).
Manu Smriti 5.35. But a man who, being duly engaged (to officiate or to dine at a sacred rite), refuses to eat meat, becomes after death an animal during twenty-one existences.
Moreover, not every non-veg food items are forbidden from consumption. Some are but not all.
Manu Smriti 5.16. (But the fish called) Pathina and (that called) Rohita may be eaten, if used for offerings to the gods or to the manes; (one may eat) likewise Ragivas, Simhatundas, and Sasalkas on all (occasions).
Manu Smriti 5.17. Let him not eat solitary or unknown beasts and birds, though they may fall under (the categories of) eatable (creatures), nor any five-toed (animals).
Manu Smriti 5.18. The porcupine, the hedgehog, the iguana, the rhinoceros, the tortoise, and the hare they declare to be eatable; likewise those (domestic animals) that have teeth in one jaw only, excepting camels.
So, it depends on what non-veg food one has consumed.
Also, the atonement measures in scriptures are prescribed based on the Varna system. So, you should specify who has consumed the non-veg food item. We need to know his Varna, whether he wears the sacred thread (a twice born) or he if he does not wear it ( a Sudra).
In general, one can do like given in the following verse once he realizes that he has taken something forbidden:
Manu Smriti 11.160. He who desires to be pure, must not eat forbidden food, and must vomit up such as he has eaten unintentionally, or quickly atone for it by (various) means of purification.
Also, for unintentional crimes, in general, the remedy is recitation of Vedic mantras. For intentional ones, atonement involves more severe penances that involve fasting for many days (like Chandrayana, Samtapana, Tapta Krikkhara, Prajapatya etc).
Manu Smriti 11.46. A sin unintentionally committed is expiated by the recitation of Vedic texts, but that which (men) in their folly commit intentionally, by various (special) penances.
So, again, it depends on whether one is initiated into the Vedas or not. If he is, then he can recite Vedic mantras like the Gayatri, which is known to clear all sort of sins.
Being clad in a pure cloth, seated on the ground and on a pure spot, and self-restrained, and by performing the achamanam with a pure hand, one should begin the recitation of the Gayatri. (212)
By the recitation of the Gayatri, all the sins, both of this world and of another, are dissipated in five nights. (213)
There is no purification of iniquitous deeds superior to the Gayatri. (214)
Verses from Samvarta Smriti.
Anyways, few penances are as follows:
Manu Smriti 11.152. But he who has eaten the food of men, whose food must not be eaten, or the leavings of women and Sudras, or forbidden flesh, shall drink barley (-gruel) during seven (days and) nights.
Manu Smriti 11.156. The atonement for partaking of (the meat of) carnivorous animals, of pigs, of camels, of cocks, of crows, of donkeys, and of human flesh, is a Tapta Krikkhra (penance)
For Sudras (one who's not wearing the sacred thread), however, the only remedy prescribed in scriptures is charity. The other atonement measures are not prescribed for him in the Smritis.
So, you need to understand that not all non-veg food items are forbidden. Even many vegetarian food items are equally forbidden.
So, we need to first find out if that non-veg food item you are talking about is at all forbidden or not. Then we can look for penances but not before that.
Quoting from the book Essence of Dharma Sindhu:
On Nanda tithis viz. Pratipaada, Shashthi, and Ekadashi, oil-baths sho uld not be resorted to; on Rikta Tithis viz. Chaturthi, Navami and Chaturdashis, hair cuts are disallowed; on Jaya Tithis viz. Triteeya, Ashtami and Trayodashi meat-eating is forbidden and on Purna Tithis viz.Panchami, Dashami and Purnimas copulation is barred.
Purnima Darsha Sankraanti C haturdashyashtameeshucha, Naraschandaala -yonousya athaila Stree maamsa sevanaat |
If a person takes up oil-woman-meat on Purnima-Darsha-Sankraanti-Dw adashi or a Shraaddha day, then he is bound to become a chandala in his next birth.
So, if consumption of meat is forbidden for all on all days, why would scriptures impose restrictions only on some particular days?