Can this be done? Can I open it in version 9.5 and save it as version 8.5?
No, here is how I would do it:
1. create a new 'Toolchest' Drawer in 9.5
2. in 9.5 copy (CTRL - Drag) all components, one at a time (objects in the Project tree) to the 'Toolchest' Drawer
3. export all variables to CSV (just in case)
4. export the 'Toolchest' drawer
5. in 8.5 create a new project
6. import the 'Toolchest' Drawer
7. copy (CTRL - Drag) all objects from the 'Toolchest' Drawer one at a time to their proper location in the Project Tree, keeping variables
If you use the variables in the 'Toolchest' you should not need to import the variable CSV file.
I have never done this with those 2 versions but I have done it with others in the past, if you don't miss anything and all goes well should not take more than half an hour or so. Good Luck!
I would do my best to get your customer to upgrade to your level first.