So the main thing he needs to do is find all his expenses for those years.

Because what is likely happening IRS gets a 1099 showing $150k revenue

But of that $150k he spent $100k on gas, vehicle maintenance. IRS lets you write off something like half of meals 50+ miles from home, etc.

But IRS doesn't know that. They just see $150k and send a massive tax bill for the full amount. He needs to adjust that and take all the deductions he can. This reduces the amount of tax, and then it reduces the amount of penalty and interest.

Make sure also they aren't reporting too much income IRS doesn't know about. If they say he has a tax debt they should send him the tax forms they prepare. Probably the part "Schedule C" is the main focus. Go over it with your dad and see it if makes senbse.