You're not misusing anything. Per Disney rules everyone in the party must get the same dining plan for the same number of days and all credits belong to the group as a whole.
All meal and snack credits for everyone on the reservation are "thrown into a pot for anyone to use in any combination they wish. There are no assigned credits per person (except adult vs kid thing)
I spoke with a CM the other day about all of this since DD is a rotten eater and we wanted to save "her credits up" towards other dining on days she is being picky. He said credits are not assigned per person: 4 adults on the regular dining plan for 8 days would receive 32 snack credits, 32 TS credits, and 32 CS credits to our cards to be used however, whenever, and by whomever we wish.
If your friends leave early, the remaining credits are for the rest of you to use because there is no such thing as "their credits" and "your credits", all credits belong to the group as a whole.
That's how he explained it to me the other day and a different CM told me the same thing when I made the reservation.
So you're not doing anything wrong by using any credits she may not use but you can verify with WDW if it would put you at ease.
Enjoy your trip!