He should not have been able to enter the second time, for 2 reasons: 1) He triggered a 10-year ban under INA 212(a)(9)(B)(i)(II) when he left after his first stay during which he accrued more than 1 year of unlawful presence (assuming the overstay was after April 1997 and he did not have a pending Extension of Stay/Change of Status application), so he should not have been able to enter again without a waiver until 10 years after he left, and 2) even if he didn't have a ban, the B2 visa he used to enter the first time would have been automatically voided under INA 222(g) if he even accrued a single day of unlawful presence, so he could not use the same visa to enter again without getting a new visa.
Anyway, to answer your question, the main issue I see is whether the Adjustment of Status was valid. A ban will also prevent Adjustment of Status. So he should not have been able to get Adjustment of Status approved unless 1) he got an immigrant waiver with I-601 based on extreme hardship to his spouse, or 2) he waited until the ban ended 10 years after his first departure before applying for AOS. You haven't told us whether he got a waiver and/or whether it was more than 10 years after the first departure that he applied for AOS, so we don't know if the AOS was correct. If the AOS approval was in error (i.e. legally inpermissible) and it was later discovered, he can be put into removal proceedings, although he might be able to file AOS again in removal proceedings if the ban is over now. Also, as another user mentioned, if the edition of the I-485 he completed asked about previous violations of status and he answered no, that could be fraud or misrepresentation of a material fact which is a lifetime ban and would require a waiver to overcome.
