Review the original item:
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 the correct way to fix the error:
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.