pls help.. my 2 cousins are fighting over an unpaid balance of postpaid internet plan

HI. i have these 2 cousins who are now having an endless fight regarding an unpaid balance for an internet postpaid plan service. cousin 1 used the name of cousin 2 to get internet plan because she is already delinquent in this certain company. after 3 months, cousin 1 failed to pay the plan and the company disconnected the service. the account which is under cousin 2's name has now an unsettled balance of about PHP30,000. cousin 2 is threatening cousin 1 that she will go after her, legally, if cousin 1 did not pay the balance. does cousin 2 have something against cousin 1 legally? cousin 1 does not have any signed agreement regarding this issue with cousin 2. could cousin 2 really go after her, legally?