If she was hearing the song in the summer, as she seems to recall, it's unlikely to be a
golden-crowned sparrow or a white-throated sparrow. They do winter in California, but should be
long gone before singing really picks up. Maybe if she heard it early in the spring? They do a little singing on their wintering grounds, but it's unlikely if it was heard all through the summer.
Flute-like is how many people describe the songs of most thrushes. It can also be hard to pinpoint where the bird is and even harder to actually see it. The problem here is the song description doesn't fit any of them. Their songs are typically descending in pitch and are not always consistent each time. Meadowlarks are also very flute-like, but the habitat of a wooded creek seems way off for where you're likely to find one.
Sorry i don't really have much of a proper suggestion for you. Identifying things from a memory can be very tricky. I've tried to ID birds from a song I didn't recognize, and even if I do it later in the same day, I often fail. If he comes around again this year, hunt that little sucker down!