If @dcj504's option doesn't work, you can also try to move your foundation out over the guitar with moveobjects off. I had an object get stuck partially under the stilted foundation while I was building with moveobjects on and couldn't get it out again, and turning the cheat off and extending the foundation over it deleted the item. You'll have to rebuy it of course, but at least it won't be weirdly stuck there.
I did try to put it back in her inventory, but alas, it did not work. She ignored the command. When I watched closely, there was a routing failure.
