Chondrolysis is defined as a generalized (all over) loss of the articular (surface) cartilage of the glenohumeral joint. The glenohumeral joint refers to both sides of the shoulder joint: the round head at the top of the humerus (upper arm bone) and the glenoid (shallow groove that functions as the shoulder socket).
The fact that the condition is generalized (rather than a local area of cartilage loss) shows that this is more than just a mechanical problem. If it were something specific (e.g., bone spur, loose cartilage, suture from surgery) that is rubbing away the surface cartilage, then only one or two bare spots would form.
But when the entire surface of the humeral head and the inner layer of cartilage in the shoulder socket are missing, then it’s clear that something else is going on. Reports of chondrolysis after arthroscopic shoulder surgery are not uncommon. The reason why this develops in some (but not all) patients is being investigated.
That may help other (future) patients but doesn’t help your sister right now. Her surgeon is the best one to answer the question of “what now”. Treatment may depend on several factors including her age, the extent of damage, and clinical presentation (her signs and symptoms, especially level of pain and loss of function).