Yes, keep it the same as your daughter.
That is what I have done, when I did childcare.
Afternoons, is usually when this age, naps.
After lunch.
How long they sleep, depends on the child.
My son who is 4, naps for 2 hours.
MY son, is 4.5 actually and he naps everyday. Still. He NEEDS it. Otherwise he turns into an over-tired fussy Troll. And he still goes to bed at night just fine.
Plan activities, in the morning.
Then have lunch.
Then wind-down.
Kids this age need to wind-down and key down, before nap.
Verbally cue them, what is coming up.
My son, will not/cannot nap, until he poops. Usually after a meal. For example.
Have a nap routine.
Make things dark, quiet, calm. Set the stage.
Then, your daughter can nap where she usually does. So she does not get out of sorts with HER routine.
The boy, can nap, in another room. You can make a place on the floor or on a sofa for him.
If it helps... tell them it is 'nap time' for MOMMY too.
That is what I do with my kids.
Then you make everything quiet.
No horse-play before nap. Otherwise, they will not key down and be too hyper to nap.
That is why, I do major activities in the morning... things using big motor skills and physical activities.
You can also do alphabets with them, workbooks etc., reading to them.
Crafts, etc.
Do you have to bathe him too?
If so, then make sure your friend knows that.
Your friend is not picking up his child until 9:00pm. If his son does not nap... then his 'bedtime' once he gets home and wound down there... will be VERY late, for a 4 year old.
Thus, having him nap, in the afternoons, will be beneficial.
Also, kids even in Kindergarten, have nap times.