Hi Edward, in your master bedroom do you have a worktable with a good chair, with a light? If not add this to the room and add to house manual.
In your house manual if you don't have it written, I would write about the great workspace guests have in that room, to spread out, make calls, and the privacy is great.
Also in the house manual under GUEST ACCESS SECTION, write the areas your guest has access to and exclude that one space your guest is now occupying.
Now what to say to current guest? If she is leaving soon, I would say nothing. If she is there for awhile, then something like "I am sorry, I forgot to mention that this space is where i do X and would you mind working in the great big space in your room or livingroom/diningroom" etcetera.
I hope this helps.
Kym Gordon-Cumbo The Slater House Pdx Portland, Oregon