No one who works at Leila's Electronics has received both a poor performance evaluation and a raise.
Evidence
No one at LE has gotten ~bad evaluation both a bad performance work at LE → or evaluation and a raise. ~raise
Lester hasn't gotten a raise. L = ~raise.
Conclusion
Lester's had a bad L = bad evaluation performance evaluation
Evaluate
The flaw in this argument, in technical terms, is that "or" is inclusive. When we say, X → Y or Z, that means that if X is true, then at least one of Y and Z is true (could be both).
Our author is interpreting "or" in exclusive terms, that you either are Y or Z, but not both.
In conversational terms, the flaw in this argument is that our author fails to consider that when we say "no one has both Y and Z", that means . She is forgetting that last option, that you might lack both.
Since Lester lacks "raise" she thinks he must have "bad evaluation", but it's possible he's had neither a raise nor a bad evaluation.
Consider how silly this would sound with a more familiar context: Nobody orders a large soda and a large milkshake. Since Denise didn't order a large soda, she must have ordered a large milkshake. (Why? couldn't she have ordered neither of those things?)
Goal
Look for a flaw that is vulnerable to the objection of "couldn't both things not be the case?", or look to match this structure: X → ~Y or ~Z X is ~Y. Thus, X is Z.