The gemara in Chulin bottom of 4a quoting the Tosefta in Pesachim chapter 2, says you may eat chametz of sinners (Jews who keep chametz over Pesach) immediately after Pesach, because you can assume they themselves don't want to eat something assur, and they will have switched their chametz with a nonjew.
The gemara suggests this idea is only necessary according to Abi Yehuda who holds Chametz She'aver alav HaPesach is asuur on a biblical level.
The She'arim Mitzuyanim Bihalacha there bring various reasons why this idea was never codified halachicaly. I'll quote some of it.
The Pnei Yehoshua suggests this point, that it is according to Rabi Yehuda who we do not rule with, is the reason the Rambam and other codifiers don't bring this down. Because we rule in accordance with Rabi Shimon and chametz she'aver alav haPesach is only assur on a rabbinical level for which we don't assume a person will not leave hetter and eat issur, and we cannot assume he switched it.
There is further discussion on this see Rosh Yosef there and Shu't Pnei Yehoshua #13.
A variant reason for the lack of this halacha is found in Beis Yosef Yo'd end of siman 115, that the gemara is only talking about a mixture of chametz that was aver haPesach. See also Chachmas Shlomo in Shulchan Aruch Orach Chaim siman 443, who studs therefore it wasn't mentioned as we already know that law. And see Yeshuos Yaakov there as well.
So in short, lihalacha, the leniency in that gemara does not apply to us.
If however you are discussing a case where you actually know the piece of real chametz your friend is offering you is the actual chametz he had over Pesach, there is no room for any discussion of any leniency whatsoever see Shulchan Aruch Orach Chaim #448 siff 3 & 5.