Because it's likely sexual harassment. In two ways:

1 - did she feel pressured to have a relationship with him? It's hard to have genuinely free consent when he can make or break her career; and

2 - sexual discrimination towards every other employee. Did she get benefits from the relationship? It's more than fair for the rest of the employees to wonder if they'd also been eg sucking his dick if they would have gotten better raises, faster promotions, etc.