As to the location, the plant is indoors on a dresser close to a window that gets light but not direct sun.
I keep all my plants outside during the summer, in fact this is when they really thrive. Things slow down considerably over the winter in large part due to the less than stellar lighting that I am able to provide. What might seem to be an adequate level of light to you (indoors) is dim to your plant.
So yes I would put it outside for the remainder of the summer. You still have not given your geographical location, so use your common sense. Jades are tender. I would not place it directly in full sun right now. When I have moved them from indoors to full sun all at once they have suffered leaf burn. Find a location that gets some sun and some shade.
do you think it has definitely been over-watered
From the information I have, yes. During the winter (indoors) I might only water once a month. The leaves go limp and flaccid but always rebound quickly when watered. I assure you, you will not kill a Jade by under-watering it unless you truly ignore it to death. Allow the soil to to go completely dry and then some before watering.
Jades are succulents and as such store water in their fleshy stems and leaves. They are adapted to go long periods without water and then when it does rain they store water until next time.
soil has little sandy/rocky appearance on top with more dirt type underneath.
A soil that is uniform throughout is preferable but I have found Jades are not that picky about their soil. The single most important thing to know with Jades is not to over-water. By the way when you do water do so copiously and repeatedly to allow the plant to re-hydrate itself.
What remedy now?
Just wait it out and see what happens. Cut any mushy parts back to firm material and just wait. I would wager that it will recover.
what else would cause to lose its leaves?
Unless there some other cause such as insect activity or disease I really don't know. My Jades are almost trouble free.
Thanks much for all your help.