I guess this could be relatively complicated. Does your friend use android or an iPhone? IPhones are a bit of a mystery to me, sadly. On android, WhatsApp backs up to Google drive. As far as I'm aware, if the app has been deleted from her mobile, the most she'll be able to recover will depend on the last time she backed up her chats. On both platforms, when you reload the app from the Play Store or the iPhone store, you will be prompted to restore previous chats as part of the process, once you've told it your mobile number. If this isn't offered, the app won't have access to backups. I don't know how a rogue message could have caused this - I've never heard of this before. If an android, this page might help:https://support.google.com/googleplay/thread/214280434/my-whatsapp-has-disappeared-and-won-t-download-again
Phil Kay ● 1046d