How can I keep elderly users safe on their computers without restricting them too much?

I'm trying to assist a few elderly individuals with their home computers. While some of them are quite tech-savvy, others really struggle. One man I've known for 15 years has a habit of clicking 'yes' on every popup, which has led to him installing numerous unnecessary and suspicious browser plugins. Every visit involves resetting 1-3 passwords due to the chaos he causes. Today, I discovered his email was being forwarded to an unfamiliar address, potentially linked to one of his kids trying to help. Given the number of questionable apps on his desktop, I'm planning to wipe everything and start fresh, but I want to ensure he can still use his computer independently. I intend to set him up as a non-admin user to limit potential damage, but I'm looking for software or options that can help keep users like him safe while still allowing them to use their devices freely.