There are two possibilities. Either:

1. The decision to lay the person off was made before the maternity leave was scheduled, in which case I’d argue she has a case for detrimental reliance, or

2. The decision to lay the person off was made after the maternity leave was scheduled, in which case a prima facie assumption is fair to make that the taking of leave obviously colored the supervisor’s evaluation and contributed to the layoff, and the burden is on the employer to prove otherwise.