SSJ contract could not be negotiated, so she left the show. They wrote in her death for the following season, supposedly several months later. Sally and unseen son died in a plane crash. Mildred left town for some reason and her sister became Mac's housekeeper and moved to a bachelor pad. As you can see, the show only lasted one more season.

There's no ep where she dies. They simply state in the first ep of season 6 that Sally died in a plane crash.

Mac, Sally, Enright, Mildred they're all great!
I don't think there is an actual death episode. From what I've read, the last episode in which she and Mildred appear is the final episode of Season 5, "Point of Law." That's also the last episode of "McMillan & Wife," then it becomes just "McMillan." Maybe their deaths are mentioned in the first episode of "McMillan," but I don't know.
You should definitely check out "Columbo," if you haven't already, I think you'd love it after watching a few. Great writing, great characterization. "McMillan & Wife," "Columbo," "McCloud," and "Hec Ramsey" were all part of NBC's "NBC Sunday Mystery Movie." Each week NBC would broadcast one of these shows and the next week a different one, so you'd have to wait a month to see the next "McMillan & Wife." That's why there are only about six episodes of "McMillan & Wife" in each season. Although I never saw "Hec Ramsey," all of the others shows were great.
Depending on her age, I'm sure your Mom would remember the opening sequence to the "NBC Sunday Mystery Movie." That's what we'd see first and it's a classic opening to some very memorable shows. Here's a link that opening for you and your Mom: