Jerry Weinberg calls this "inflicting help".
It's swell that you did all this work, but there's a quality problem here. A customer has a right to get what he asks for. "More than what he asks for" can be just as bad as "less than what he asks for". If I order a Geo Metro, and the dealer delivers a Cadillac, the dealer may think himself very nice--but my garage only fits the Metro. That would be bad enough, but I now have a bill for the Cadillac, and I have to rebuild my garage. So I'm not happy. The dealer wanted to give me a better car... but I didn't want that. I wanted what I asked for.
Similarly, if I ask someone to install a DVD drive, I expect two things: 1) A DVD drive installed; and 2) a bill for a DVD drive installation. And no more. Not an operating system upgrade, not attaching the main computer to the network. (A computer that isn't on the network is much less likely to be hacked than one that IS on the network. If security is the issue, they may be better off than you think.)
Quality is all about providing value to some person. Your problem is that your customer doesn't value the work you've done, mostly because you apparently did it before they understood it. They do understand money in simpleterms, which is why they read your invoices so carefully. They GET that.
You can indeed lead a horse to water. He may drink or he may not, but that's up to the horse. I would suggest that you've crammed a firehose down his gullet and told him to get ready to swallow.
Next time, make your observations, start with a one-pager (not four, not forty). Explain the risks, the costs of doing the work, and the costs of not doing it. Then wait for them to ask you for help.
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