Divorce/Filing Status Issue

Hey There,

I have a client with an interesting tax issue. Some years ago (2015) my client and his wife decided to divorce for financial reasons (only). Therefore, they engaged and paid an attorney to file for divorce. It was their understanding that all divorce documents had been properly filed the divorce was finalized. Therefore, they have filed their returns with the filing status as SINGLE/HOH since 2015. Unfortunately, they just discovered that the attorney never filed the proper paperwork then their divorce was indeed never finalized. They are indeed working on correcting this snafu. So clearly, what I want to determine is what action, if any, to take for his previous years returns regarding the filing status. I have done some initial research but nothing has popped up. Any thoughts? Any seasoned researchers out there? I am looking for advise/opinions based on primary authority (ie, code, regs, TAMs, PLRs, court cases, etc...) rather than the character of my client. Thank you!!!