How low have mains frequencies gone? What areas once used 30 Hz, and why?
The historical reasons for using 30 Hz and other frequencies is documented in the book Electrical Engineering Papers by Benjamin G. Lamme, published in 1919. In the article 'Story of the Frequencies' he explains that large generators and those powered directly by reciprocating engines ran at lower rpm, and therefore preferred a lower output frequency to reduce the number of poles required in the generator. Lower frequency was also preferred for long distance power transmission.        

Finding all the areas that once used 30 Hz would take far more intensive research than this question deserves. My Google searches came up with nothing, however I did find some information that may be relevant.  

In the 1880's electric power was used mostly for lighting, which preferred higher frequencies to reduce flicker, so frequencies such as 133&#8532; Hz and 125 Hz were common. These higher frequencies enabled the use of smaller transformers. However the AC motors developed at that time required  frequencies as low as 16&#8532; Hz. This was a problem because converting from one frequency to another was difficult. So different frequencies were usually generated for residential and industrial use, with many factories having their own power plants free to run at whatever frequency they desired.  

Around 1890 the 'compromise' frequency of 60 Hz was introduced, which eventually became a nationwide standard. However 25 Hz was still used in some places until quite recently. 

In the article Early Electrification of Buffalo, it is mentioned that Westinghouse had adopted 60 Hz for lighting and 30 Hz for power, but for the Niagara Falls project they settled on 25 Hz. That service continued until 2006. Several other hydro stations on the Niagara river also ran at 25 Hz. I don't know if Detroit used Niagara power in the 1940's or 1950's, but it seems likely that at least some parts of Michigan did.   

Sue Simkin might have lived an area which had a local power plant running at 30 Hz, perhaps attached to a factory. This could have been anywhere in the US. With so many private power plants involved I think it would be hard to rule out the use of 30 Hz in any area.