I know nothing about these RFCs, but this seems trivial to clear up with a reference to the text. Here's RFC6409 section 8.8: https://tools.ietf.org/html/rfc6409#section-8.8. There is exactly one "MUST NOT" and it looks like it only prohibits what your parent said it does.
There seems to be an idea floating around that anything at all is permitted under the RFC for MSAs unless it is specifically prohibited using the phrase "must not", regardless of context and other guidance in this RFP and the other RFPs that refer to it. Perhaps this is true. I have a sincere question for anyone holding this opinion: would Google be in compliance if their software altered the message body by adding the sentence "Google is the best search engine ever." here and there, at random? If not, why not, exactly?
In Poland it was standard to add ads as mail footers among homegrown free providers. Some still do. If they don't have sold ads they just advertise themselves.
Made me hate pretty much all footers, especially the bloated corporate ones.
