Mom is getting screwed by Ex-husband

My step-dad, now my mother’s ex husband, owes a shit ton of back taxes to the IRS. I believe the amount is around 67k in back taxes. Him and my mother filed jointly back in 2016, she did not know there were any issues with the taxes back then.

Fast forward, he gets in trouble, puts a lien on the house for the taxes, puts the house in my mothers name, they get divorced, he says in the divorce papers that the taxes are on him, not her.

She’s been fighting this since the divorce, but the IRS won’t stop going after her. She talks to her CPA and he files an innocent spouse form.

Fast forward again, while waiting for this innocent spouse form to process, the interest on the taxes have brought the grand total to over 100k. She gets a letter from the IRS about garnishing wages. So she ends up getting a meeting with an IRS agent. Come to find out, the CPA filed an INJURED spouse form. So this whole time, this dude files the wrong form and while we were waiting to hear the ruling on the innocent spouse form that was never filed, interest is stacking up.

My question is, is the CPA liable for the accrued interest since he made a big mistake? And my 2nd question is, can the IRS amend the return and honor the innocent spouse form since it’s been passed the statute of limitations?

I’m not great with tax lingo, so I apologize if this is a tough read.