I think the bigger issue here is you need an IT technician to assist with your mother's day to day requirements. It's clear that there is confusion and conflation between IT and ISP Tech support roles here.. An ISP is not there to be IT tech support. You don't call the water utility when you have a leak in your home, you call a plumber. It's important to remember there are distinctions in the roles and services provided.
The need to flush caches on your computer points to a local issue on your computer - Afrihost's tech determined that there was perhaps a DNS cache issue (maybe aged DNS records) and this needed to be cleared. For the the ISP technician to reach this conclusion, it's certain they would have confirmed the line was in working order and that the router was in working order. This is what they check first. So at this point, they are not blaming your equipment, they are trying to assist in determining what could be wrong, and for this, they need, at the very least, your ability to type and run basic troubleshooting on your own equipment.
The ISP's job is to ensure your fibre internet service works well to the handover point, which is the router, everything past that (your devices, TVs and computers are not their problem). In my opinion as someone with experience in both IT and ISP, the technician went above and beyond, determined that the issue was not router/internet related and attempted to assist further with IT related support, instead of just telling you to go seek assistance elsewhere. Now if this doesn't work for you, I suggest you ask an IT technician to look at your PC, and not reach out to your ISP for IT support.
Regarding your comment about the potential for fraud, this exists everywhere and requires diligence and education. I too have an older mother who is always online, and its forced me to take time to teach her these risks. However, in this case, your mother initiated the call to Afrihost and reached out for assistance. The technician likely tried his/her very best to assist, especially out of respect for her, the same way we all do. Once again, to avoid the risk of malicious actors, appointing a trusted IT technician to act as your mother's intermediary for all things tech to will eliminate these risks. However blaming the ISP for a call initiated by your mother is somewhat off field.