Mother taking father's Ativan pills.?

I need some input/guidance on a problem. My mother has been an alcoholic for years. It has recently come to my attention that there is a very good possibility that she is now taking my father's prescription Ativan pills. My father (who is doing extremely well & being weaned from the meds) disclosed to me last night that my mother asked his psychiatrist to refill his Ativan despite the fact that he hasn't taken even one over the last month. Last night my mother was not only drunk but seemed to be on something. She was incoherent, confused, was experiencing memory problems, and had difficulty with movements and coordination. It was quite disturbing. I am not sure of my father's intention to report this to his psychiatrist, but apparently he has an appt. with his doc on Thursday. Just wondering if it would even be worthwhile to intervene and give the doc a heads up or if I should wait to see if he tells him himself. My dad has a history of hiding things that he's uncomfortable dealing with.