It's never too late to file. If your father in law was entitled to a refund, it is too late to get it paid.
The more important questions are, was a return required at all (based on income and other factors), and who was the responsible person. The responsible person to file a tax return might have been his child, but might not have been.
To determine if a return was required at all, you can start by reviewing the 2017 instructions for form 1040, starting on page 4.
https://www.irs.gov/pub/irs-prior/p17--2017.pdf
If a return is required, you will have to see a tax professional who is willing to prepare a 2017 return, or do it by hand using the 2017 forms and instructions from the IRS web site. Turbotax is not supported or available for purchase that far back.
