You may want to look into the
shadow command. There are a few options that must be set for it to work and an option that must be turned off to prevent the session you're viewing from being prompted. See this Microsoft help article: "How to Shadow a Terminal Server Session Without Prompt for Approval" as well as Microsoft KB article 278845.
EDIT: I do admit that this is not as user-friendly as some other solutions. However, if you're dead set against installing anything new on the server it would work. For friendlier options, check out TeamViewer, CrossLoop or even Bomgar gear.
EDIT #2: In light of new information, since you want to have someone that is physically at the server see what you're doing you have several options. you can use a third party tool like teamviewer. However, if you don't want to install anything new, the person that you're instructing will have to log into the server and then you'll need to remote into the server with Remote Desktop and then use the Shadow command on session 0. A final option is if you have some kind of remote management card such as HP's iLO.