Or maybe somehow someone managed to enable WhatsApp web on her phone, or she used it on a public computer and forgot to turn it off? Otherwise, completely out of ideas.
This might be a bit esoteric but is WhatsApp the only such problems she's had?
On Reddit there is this group-stalking phenomenon where if someone (of limited maturity but rich in online contacts who seem to have nothing better to do with their time) for example disagrees with your post, they try to get their friends to get mods to ban you from subreddits by complaining of spam from your account via their inbox/messenger.
Failing that, they usually resort to bombarding Reddit with "concerns" about "wellbeing" which results in inboxes getting cluttered with "A concerned Reddit user reached out to us about your wellbeing" ...
I'm not saying it's that, but human spammers tend to work in networks that respond viciously to anything that messes with accounts they've set up as part of their networks, so if she reported them as spam and they somehow zeroed in on her or suspected her as one of the few possible causes of one of their accounts being flagged for spamming, perhaps they collectively retaliated against her out of spite?
