Are you both using the same type of browser (IE or Firefox etc)? I think RH output should be compatible with either but it's something to consider...
The only other thing I can think of is that it is running slowly and the TOC is not loading up before patience runs out?Are you both accessing it from the same physical location?
Does your colleague get the top toolbar with the choices for Contents/Index/Search (assuming you have thse set up in your output)? Try switching to the Index and back again..
My colleague was using his preferred browser, Chrome. This does not seem to support the Contents.
He is able to open the WebHelp in Internet Explorer and FireFox.
Thanks for your suggestions.
Fellow Adobe Community Expert Peter Grainge has some info at this link that will help with Chrome.
You will be interested in Snippet 103.