You don’t fire someone in this position unless the investigation turned up something. This is mutual fund management, not politics or Hollywood. Fidelity is a private company run by a woman in a liberal city . . . not smart to even think about it there.
What they have to show is that
1) they had a romantic/sexual relationship
2) he is in a superior position to her in the same work area, not necessarily direct management authority
They were both in the research area. The relationship may have been entirely consensual and based in mutual interest, but since it could be construed as a quid pro quo, it opens the firm to liability if they do not take action.