Cancelling m-net contract

[apophis 2]

A few days ago, they switched the lines from copper to optic and people in my flat got an option of 
getting a new contract on the faster optic lines. Did so as well, but in my case, the situation is a bit different.

The previous tenant had a contract with them on the copper lines. After moving in, she suggested that instead 
of cancelling and renewing the contract, it would be easier to change the IBAN from which they subtract monthly payments,
from her to mine. Did so and this worked well for a few years. 

The sales representative of m-net, upon hearing this, suggested "forgetting about" the old/her contract since it's not mine and 
ignore it. They will eventually contact her and sort this out. Moreover, he also suggested to cancel the payment for the last months.
Did so with the month of September. 

Fast forward to Yesterday. Got a mail from them to pay for September the original amount and a Rücklast. 

After reaching out to tel customer support, they mentioned that only she could cancel the contract. But, by the looks of it,
they have no means of reaching her (they only know her address which is now my address, no phone/mail).

Does anyone have "mnet experience"? 

Is it alright to keep "ignoring" their letters? What happens if they can't reach her? Will the amount and incurring penalties be paid by me?
or will the contract simply be cancelled due to lack of payment. 

Haven't found anything in my new contract related to the last previous point.