Who is in the right here?
Person A: a professional service provider, texts (SMS rather than WhatsApp) their client at 8am on the day of the appt advising the slot (which was for 6pm) is no longer available and asking them to come at 8pm that day or rearrange to a different day.
Person B: the client, has notifications turned off for SMSs on their phone as they get annoyed with getting lots of SMSs about parcel deliveries and from scammers. They don't see the message and turn up at the appointment at the original time to find noone there.
This was only the 2nd appt and Person A has previously communicated only by EMAIL.
VOTE:
YABU = Person A is correct. Person B should check SMSs, especially if they have an appointment.
YANBU = Person B is correct. Person A should have PHONED Person B to ensure they got the message, especially when they didn't respond to the text.