I can tell you that, despite their threatening calls/letters, the IRS is actually willing to be flexible and to work with people who are in tough situations. They will settle for less than the owed tax but they want to do that with people who are cooperative, who admit they owe money but can't pay it, etc. Divorce is a classic problem, economy is another, being laid off is another, etc.
I wonder if he could get help from an accounting/business department or a tax law department at a college or law school? They might do his returns for him. Or he could pay a student a reasonable fee.
If he owns up to his ex not doing it, and says "How can you help me and what can I do?" the IRS will be initially difficult and ultimately cooperative. They will be better off getting something, anything, late from him than the current "nothing at all". Not sure how long this goes back, but they don't keep pursuing debts that are more than 10 years old.
He should go to/call his local IRS office (and the state tax office, if that's involved too) and see what he can do. If he doesn't find cooperation, he should go to the next supervisory level. They will soften after initially stonewalling him. They will appreciate his efforts to make things right.
The worst thing he can do is ignore this and let it get worse.
They will work out a payment plan for a portion of what is owed, and hopefully help him prevent this from continuing with subsequent tax years. Tell him not to be afraid, that SO MANY people are in this situation, and it CAN be worked out if he shows he is cooperative. The IRS knows they cannot get blood from a stone - if he doesn't have the money, he doesn't have it. They can also help protect him against future actions by his ex.
Good luck.