Assuming the 1099-R is correct, and there was no named benificuary of the retirement account or the estate was the named beneficiary (if there was a named benificuary, then contact the retirement plan administrator.)
The estate administrator would have to report that on the estates 1041 estate tax return and issue K-1's to the beneficiaries of the estate.
Suggest that you read IRS Pub 559 (Survivors, Executors, and Administrators) for a lot of good information about filing the final return and estate return and other requirements.
https://www.irs.gov/pub/irs-pdf/p559.pdf
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
