Your webhelp output (I assume this is what you are producing) is in fact just a collection of HTM files so there is nothing to stop them from manually editing them. However the content will not be indexed in the search. Perhaps more worrying is content that is returned by a search may not be actually available. The only way to get around this is to recompile which would need a RH licence.
A method some of my colleages use is as follows: Add a blank topic in the TOC with the name of the customer. On first delivery (the WebHelp runs on a webserver of the client), the empty topic is delivered with the output. With every new iteration of the help, this blank file is left out of the help output. (My collegue manually deletes the file after generation.) This way this file is never overwritten. The client adds links, text, etc in these topics in the output. The downside: The topic is not indexed and I don't believe the search picks up the topics content. But it made a few clients happy...
I have just tried this out and it works fine! This is great. Thanks so much. I'm sure our customers will be happy now.
See www.grainge.org for RoboHelp and Authoring tips