I am not aware that the IRS or state, or anyone has come calling. It sounds like the wife washed her hands of this years ago but implied that their marriage was on the line if he didnt square up with the IRS and such. 

If I agreed to do the tax return for three years how much per return would you charge?

I also do bookkeeping but I am not super well versed in payroll or dealing with such extensive back taxes. I certainly feel like I could figure it out but I dont want to open a can of worms for him that I cant help him wrap up? Is the expectation that we prove what can be proved and he just swallow the rest since his records are so incomplete? Once I submit this, if we find he owes tens of thousands, will he just go to someone or the IRS to make an offer in compromise? 

How much would you charge per year for the bookkeeping? I was thinking $1-2000 per year? plus about $4-600 per tax return. Maybe this is too much....I just know its going to be a lot of trouble.