You can tell him his returns were not correct and advise him that they are still open to IRS audit.  If he asks how likely it is that IRS will discover the mistake, you can point out the percentage of returns that IRS audits in his income range.  If he wants to know the difference between self-reporting and waiting for a possible audit, you can explain the 20% negligence penalty.  As far as recommending an action, isn't that more of a moral choice than professional advice?  Of course, for future returns you want to bring him back into compliance.

Married men who cheat on their taxes are more likely to cheat on their wives, also.  The dilemma might resolve itself when his future ex-wife turns him in.