maybe they had their log in details saved on the old phone so they would be automatically logged in, but forgot what their sign in details were to access their account on the new phone.
New phone. Yes I'm logged in on many other things but my mobile goes everywhere so if I feel like a natter when supping a coffee in the village I can, except I couldn't as phone just would not accept password. Old phone did but carrying 2 phones got a bit wearisome
Sorted now.