damit I wanted to say telegram!!!!
seriously though it sounds like there is a wonky setting inside whatsapp or there is a dupilcate account on the phone, that does cause the issue from time to time, the wifi might be a symtomb of something else.
easiest fix I would sugjest uninstall whatsapp (leave the data), remova all accounts, and then add the google account and whatsapp again, might fix your problem.
does other things work on the wifi?
is it only one wifi network in question where the whatsapp does not work? have you tried making a hotspot on your phone and seeing if she gets messages then?