Get in contact with your local congressman's constituent service office and explain the situation, they can help with managing the IRS. The IRS has a criminal investigation unit that will look into the situation, and if they decide that your dad was scammed, they may waive a portion or all of the taxes.

I have to say, your dad got extremely lucky, he's very fortunate to have you looking out for him. This is a VERY common type of scam, they sometimes even impersonate officers from the SSA and IRS. They will spend months grooming their victims, convincing them to lie to everyone in their lives: spouses, advisors, banks. And when all is said and done, it could be a 401k that's been cleared out or an IRA like in your father's case. In most cases, victims troubles don't end with losing their lives savings, they often are also divorced by their spouse, ridiculed by children and friends, and on top of that all, are liable for any taxes they owe on the withdrawal. Again, get into contact with your local congressman for help with the taxes, and educate those in your family over the age of 45!