Like others have mentioned:
1) What is the current age(s) of the individuals?
2) Are they planning on taking Social Security early, on time, or postponing for a larger benefit?
3) What is the health insurance situation?
4) Do they have mission plans? If so, where and what is the expected cost?
5) What is their spending habits?
I would highly recommend using a retirement calculator, here are couple after a quick search:
As someone who is reasonably sophisticated in financial matters, I would have a serious conversation about retirement and expectations so that they can make sure that this is a realistic scenario before they quit their job and make other decisions which could adversely affect them.
Best of luck!