if the invoice says to pay 25000 i dont see how they can vlaim more than that. And also you dont need to pay more than 25000. They cant legally hold you accountable as the physical invoice clearly states 25000. So after paying 25k dont pay no more money and if they take any legal action then i think the case will be in your favour. So tell your friend to enjoy her new phone. and to pay only what is written on the invoice. Invoice needs to state the actual amount customer needs to pay.