I have asked the same question, but I am not sure. The act does not specify that the cap is limited to registered users only, so still confused.
However, I said this in the beginning (where many were angry and called me scaremongering and trolling) and I will say it again.
There is nothing we can do about the e-Toll system. It has been implemented and users of these roads will have to pay up when using them. Government has something to their advantage - the law. No matter how unjust or bad the law, it still is the law and everyone will abide. People will be able to get around the law for only so long before it catches up.
If you are a daily user of toll roads and do not want to take back roads, the best thing to do is to register. Pay the fees due and you will save yourself a lot of money. Like, in your example, if your friend registered, he/she will only be paying R450 per month instead of R1200.