If the fault is down to her equipment then I would brace yourself as she is likely to get billed £130 for Openreach attending and determining that the fault was with her equipment.
I would probably try to talk my way out of that. Vulnerable customer, not reasonable to expect them to understand that the fault could be with the phones or how to diagnose a fault. etc Keep it simple though, no embellishment. The problem is that the OP tried to diagnose the fault, so depending on which way you want to go with that they may have to admit some form of responsibility. The key is to make your complaint seem reasonable, using statements that are true.
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I would probably try to talk my way out of that. Vulnerable customer, not reasonable to expect them to understand that the fault could be with the phones or how to diagnose a fault. etc Keep it simple though, no embellishment. The problem is that the OP tried to diagnose the fault, so depending on which way you want to go with that they may have to admit some form of responsibility. The key is to make your complaint seem reasonable, using statements that are true.