You may dissaprove. If necessary, give her a reminder letter to state it clearly that
1. She needs to compromise with her employer on which days to have day-offs, and in this regard, be co-operative!
2. State specially the occasion that you have juz mentioned here. Statin as well clearly that the past Sat had been counted as day-off already due to the fact that you have informed her well in advance, and thus no more rest day will be granted to her to compensate it.
This helps you with further arguments that may come in future.