I listen to my body and have a rest day as and when it seems necessary, or when I've got other stuff going on that day. Usually every 6-10 days.
I do some form of full-body workout most days, so if she is doing legs, I'll add in a short arms workout, plus some abs. Or a Growingannanas HIIT session. And always always always a bunch of planks. And a dozen or so pushups.
If I've done, say, a really tough legs workout, I'll give legs a miss the next day and do a bit more cardio/HIIT. Ditto arms.
Basically I play it by ear. After 4.5 years of doing this, I am pretty attuned to what's works for me.