If T1 is using PAYG 'bundles', she might be getting caught out by the way that long texts are handled by modern smartphones.
'Bundles' normally include sending loads of 'plain' (SMS) texts, as well as making calls and using data, but they don't include sending picture (MMS) messages. There's a character limit on SMS texts, beyong which (even though there's no picture involved at all) a long text message gets converted to a picture message, which needs additional payment (beyond what's included in 'bundles') before it can be sent.
Ask T1 to try sending a very short text message (such as 'Hello'). If that goes through, but long messages don't, it will be because she needs to add some non-bundle credit to her account.