1099S is for the sale of real estate, what did he sell?

Id venture to guess an amended return could help whittle that balance due down, since basis hasn't been accounted for on the 1099S.

Thats a rather long time for IRS to wait to send a notice about 2018, has this been going on for awhile and he ignored them until he finally decided to ask someone for help?  He may be in a tax court appeal situation already and might have to refer him out to someone experienced in that.