It's against pretty much every company policy to have a relationship with someone who reports to you.
All 42,000 employees should be held to the same standard, otherwise you will end up with all kinds of problems within the company.
Company leadership should lead by example as well.
Right, and to make it explicit for some people following along who are wondering why companies have that policy... It's to prevent predatory and abuses of power. There's a power imbalance that can easily lead to exploitation.
The reason companies care about this so much is more practical than moral. Bosses having sexual/romantic relationships with employees leads to irrational decisions that are bad for the business.
Adding on to that clarification: The leverage the boss has over the employee creates a rational pressure for the employee to ignore any boundary they might have otherwise had in order to avoid any chance of the boss using that leverage against them.
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.