i need help please

my mum got a facebook message from a friend in the middle of a conversation asking her to call but when mum called her, her friend said she never sent that message.
so now mums worried about being hacked and shes staying off facebook. somethings going on but we dont know what. i looked up stuff about facebook scams but didnt find anything about getting a message from a friends account but its not actually from them and asking you to call them.

(edit: everyone just keeps saying to change her password but we already did that and it isnt enough anyway, because the bad thing has already happened and we arent sure what it even was. that what we are trying to deal with)

i dont know anything about hacking or why they might have wanted mum to call her friend, like did that give them access to both peoples contacts, or everything on their phone? thats the kind of thing mums worried about now. shes still worried because she did what the scammers wanted, to call her friend.

help meeee. please :)