She has demonstrated some sound strategic thinking. When you are making a career transition, it is usually advisable to set up as few hurdles as possible. In this case, for example, she is looking for a new employer in a new industry, but in a similar type of job. Thus, she has a visible connection to her next employer (kind of job) and two hurdles to leap (employer - industry). Two hurdles are easier than three and one strong connection is better than none.
A strong opening is a good idea. Her clearest strength is her experience in customer service and she lets the reader know about it right away.
The second paragraph highlights positive characteristics she has shown in her professional experience that an employer needs. She has shown her business sense by referring to customers, profitability and not taking time away from line managers. In addition, she addressed a potential credibility gap by showing her ability to quickly master knowledge of new products.
Her third paragraph is especially important because she indicates why she wants to work for this particular employer (Handy-Wrench). The fact that she researched the firm also shows a professional mind-set.
What if she writes a letter after her last day of work at her? No problem.
Most of the letter can stay the same. In the second paragraph she can refer to "a business much like yours" instead of writing "my current employer."
When writing to other firms about her interest in the same field, only the third paragraph will need substantial change. If she has done her research in this industry and field, she should be able to identify something to say about each particular firm without an enormous amount of additional research.
New material : Your cover letter can include material of interest to an employer that would be difficult to present in a resume. Your motivation for wanting to work for that particular company is an example.