The business of borrowing someone else's property without permission is a bad trait and should be a "red" flag to any relationship.
Your partner needs to replace your 3-year-old Sony RX100, Mark III with one which is in working order. That camera new, sold for around $700. That camera is a good camera, however, it is still a point and shoot camera although with one of the largest sensors in that category (1 inch)
He can replace it with one he purchases on KEH.com and those cameras are guaranteed to be in working
This issue needs to be resolved as soon as possible. If your mate is not willing to"make you whole", then you have some new insight into their basic ethics, charicter, loyalty and integrity as a person. These four traits are more important than any camera or other property
If he does agree to replace your camera with the same model he lost, then all should be well, however, you need to be assured that in the future that this kind of "borrowing" is not done again.
I have a strict rule when it comes to my cameras. NO one is allowed to use them unless two criteria are met
!) they are a competent photographer in their own right and
2) they have the necessary assets to cover any loss while they have the camera
Good luck dealing with this learning experience. However it turns out, you have not had to pay too much for this class in the school of "hard knocks"