Blacklisted kay PLDT ang name ng ate mo, meaning, di na siya makakapag-apply ng any services/plan sa kanila using her name.
Credit record [ng ate mo] may be affected as well.
I've heard it from someone working in a unibank. They use credit bureau data during the loan processing period. Risk appetite varies from bank to bank but the general rule is they adhere to the given credit scoring guide as basis for approvals. Even an unpaid loan or credit card debt 10 years or older is enough to get your application cancelled.
Once na nasa delinquency list na ng Credit Bureau ang name mo (in this case, name ng ate mo), kahit pa ma-settle lahat at mabayaran lahat, the record will not be deleted. It will stay there forever at pinagbabasehan yun ng credit institutions (banks included) for credit profiling ng mga applicants nila (for loans and credits).
Fortunately, walng kaso yan since wala pang batas tungkol diyan. So walang hit yan sa NBI record mo. Unless habulin ka personally ni PLDT, which I doubt they will since di sila mag-aaksaya ng panahon at resources sa lawsuit sa iisang individual. Usually ang ginagawa nila diyan, binebenta na lang ni PLDT yung credit mo sa 3rd-party collection agency partner nila. Sila na ang may responsiblidad na kulitin ka (through calls, texts messages, and emails) tungkol sa "utang" mo kay PLDT.
Once this happens, you know na tarnished at nadungisan na ang credit record mo (in this case, credit record ng ate mo).
And I've also heard (although still unconfirmed) na chine-check din ng immigration ang credit record mo. So kung may balak kayo (in the future) na mangibang-bansa, be prepared na kakalkalin ng immigration ang credit record mo. (Again, this is still an unconfirmed info)