Praise be to Allah.
Allah, may He be exalted, says (interpretation of the meaning):
“Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful”
So what must be done when offering expiation for breaking an oath is to feed ten poor or needy persons.
Ibn Qudaamah (may Allah have mercy on him) said:
The one who is offering expiation either finds the right number of poor persons, or he does not.
If he finds them, it is not acceptable for him to feed fewer than ten in the case of offering expiation for breaking an oath.… This was stated by ash-Shaafa‘i and Abu’th-Thawr; al-Awzaa‘i regarded it as permissible to give [the food] to one person…
Ashaab ar-ra’y said: It is permissible to give the food to one poor person over ten days, if it is expiation for breaking an oath … And it is not permissible to give it to him all on one day. Abu’l-Khattaab narrated this from Ahmad. That is because on each day, he will have fed one poor person by giving him the amount of food prescribed, so that is good enough, as good as if he gave it to someone else. And if he were to feed the same poor person because of another expiation, that would also be good enough; therefore, by the same token, it will be good enough if he feeds him because of this expiation.
Allah, may He be exalted, says: “So its expiation is the feeding of ten needy people.” The one who feeds one person has not fed ten, so he has not complied with the command and that is not acceptable. Moreover, Allah, may He be exalted, has ordained that the expiation should be feeding ten poor persons, so if he does not feed ten people, then he has not offered expiation as required…
End quote from al-Mughni (13/513).
Based on that, with regard to this poor person who took two meals, claiming that one of them was for his friend, one of two scenarios must apply. The first scenario is that you think it most likely that the man was telling the truth, in which case you have fed ten people, because one of the ways in which Islamic teachings make things easier for people is that what one thinks most likely to be the case may be regarded as equivalent to what is certain, when it is not possible to be certain.
Abu ‘Abdillah al-Muqqari (may Allah have mercy on him) said:
What matters with regard to establishing causes and innocence, and anything on which rulings may be based, is certain knowledge. But if that is not possible, then what is most likely to be the case may be accepted as certain knowledge, because it is close enough to certain knowledge.
End quote from al-Qawaa‘id (1/289).
The second scenario is if you do not think it most likely that he was telling the truth, and you do not believe what he said. This is what appears to be the case from your question. In this case you must give another meal to another poor person, so that you can be certain that you have fed ten poor persons, and so as to dispel any doubts from your mind.
The Permanent Committee for Academic Research and Ifta’ was asked:
Is it permissible to give expiation for breaking an oath to one poor person, or must it be given to ten different poor persons?
They replied: If expiation for breaking an oath is given in the form of food, it must be given to ten poor persons, giving each poor person half a saa‘ of food. It is not acceptable to give it only to one poor person, even if you give him something every day for ten days, because this is contrary to what is mentioned in the text.
Permanent Committee for Academic Research and Ifta’
Bakr ibn ‘Abdillah Abu Zayd, Saalih ibn Fawzaan al-Fawzaan, ‘Abd al-‘Azeez ibn ‘Abdillah Aal ash-Shaykh, ‘Abd al-‘Azeez ibn ‘Abdillah ibn Baaz.
End quote from Fataawa al-Lajnah ad-Daa’imah (23/21).
And Allah knows best.