When did she get divorced? Did she have any other income? Maybe she didn't have enough income to file. Hopefully the ex didn't file her on his return as a Joint return. Sounds like you or she needs to get past copies from the ex or the IRS to see what was filed. You may need to amend the prior years.
You can request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript
For a fee of $43 you can get a complete federal tax return from the IRS by completing Form 4506
https://www.irs.gov/pub/irs-pdf/f4506.pdf
