Is your wife planning on discussing this in person first and then submitting a letter? I've found that if you resign and they ask the reason, you can tell them you are looking to spend more time caring for family. If they want to keep you, many times they'll offer part time hours or a work from home arrangement during the in-person discussion of the resignation. If they don't offer first, she can always say "but i really enjoy working with the team, and if you ever need someone to pickup some work from home I'd love to help." This isn't pushy, leaves the door open, and really demonstrates an employee's dedication to their job even in the midst of a resignation. If they seem open to this, even if a work arrangement isn't officially agreed upon, she can then work it into the resignation letter by reiterating her offer.
As a previous manager, I would not recommend just turning in a letter before having an in person resignation with her supervisor. And I wouldn't plan on turning the letter over right away at the meeting because you may want to adjust it based on how that conversation goes. The letter can be sent afterwards. Just sending in a letter without telling them first would be shocking and probably won't go over well.