Tell her that its not important of who is right or wrong, but that her precious bundle of joy stay health! Tell her you've read information for your own knowledge and thought she might be interested in reading it.
https://kidshealth.org/parent/pregnancy_newborn/formulafeed/formulafeed_storing.html#
Specifically stated in the article....
"How long can mixed formula keep in the fridge?
You should always refrigerate any bottles you fill for later feedings to prevent bacteria from growing, as well as any open containers of ready-to-feed or concentrate formula. Throw away any mixed formula after 24 hours and any open ready-to-feed or concentrate formula after 48 hours.
How long can a bottle keep at room temperature?
Discard any prepared or ready-to-feed formula that's been sitting out after 1 hour.
If formula is left over, can I offer it again?
No, throw away any leftover formula. There's a chance bacteria may have formed since the last feeding, which could make your baby sick. "
Hope that helps! If at all possible, tell her you made that mistake and someone informed you luckily before your little one got sick.. fib a little to spare HER little one getting sick