Take a look at the original sentence again:
Lorna did not last long in Mr. Wolcott's busy office,
her long fingernails made accurate typing impossible, and her abrasive
manner scared away too many potential clients.
You chose to fix it the correct way:
Lorna did not last long in Mr. Wolcott's busy office.
Her long fingernails made accurate typing impossible, and her abrasive
manner scared away too many potential clients.
Lorna did not last long in Mr. Wolcott's busy office is a main clause. Her long fingernails made accurate typing impossible is the second main clause. You are always correct when you use a period and a capital letter between the two main clauses that create the comma splice.