Having run communities myself, I would: take it seriously—immediately—let the community know that harassment is unacceptable, support the person who reported it, investigate privately, remain neutral to all parties. Based on the result of the investigation: enforce consequences or changes—nothing, warning, timeout, ban—introduce or amend policy to help prevent it happening again, support healing of the victim and the community. These are all quite possible in private/managed communities. Not a panacea by any means, but worth doing.

Everyone who works on any side project should always feel free to stop whenever they dang well please.

Author here. It was my full-time job, not a side project. I've edited the post to make that clear.