Most Helpful Guy
If she has warm Michigan winter clothes and coats, she should take them. SF can get pretty chilly. It's not entirely about temperature though. It can get wet and foggy. That can chill you to the bones in the winter if you are out in it for very long. 35-40 degrees and damp can feel colder than 20 degrees and dry.
Also, I assume she won't just be in SF. She'll probably end up going to other areas, like up in the mountains. The Sierra can have as much snow as northern Michigan.
Even in the summer, going out by the ocean can get pretty chilly. People who live there know to always take a jacket or something when they go to the ocean - year round.
If she's limited on room, she can get by without the down coat. She can maybe get by without the long johns also. If she needs them she can send for them later. I only lived in the Bay Area a short time and it was summer. But I lived east of there in Sacramento for a long time. I wore a full set of thermals all winter, and sometimes found myself wearing a full winter coat even when I wasn't going to the mountain.