From the form instructions:

Extension for executors filing to
elect portability.
Executors who did
not have a filing requirement under
section 6018(a), but failed to timely file
Form 706 to make the portability
election, may be eligible for an
extension under Rev. Proc. 2017–34,
2017–26 I.R.B. 1282. Executors filing to
elect portability, may now file Form 706
on or before the later of January 2, 2018
or the second anniversary of the
decedent’s death. For more information
see
Rev. Proc. 2017–34.

I would explain the possibilities to the family and leave the decision to them. They 706 for portability is simple, however I know some EAs charge the full fee.