Steve wrote:
My sister has she would like to lock partially open - a typical glass
sliding patio door - except it appears to have been installed
backwards!

How about putting a standard chain latch on it.
The kind that lets you open a door to see who is outside without
letting them get in to easily.
Some of the ones that are a single solid loop of steal that hooks over
a hook on the door is pretty strong.
If it was installed right at the top of the door an intruder may not
even find it and waste all his time trying to find what is holding the
door.