Is it permissible for me to prevent the maid from going to our neighbor's husband's house after he tried to assault her, and must I inform the wife of what happened upon her return?

It is not permissible for a maid to be alone with a man in his house, due to the potential for corruption that could result. As for what has occurred, one must think well of a Muslim and not accuse them without clear proof. If the maid is a righteous Muslim, and the husband is not upright, then his wife can be advised to investigate the matter and inform her of what the maid conveyed. However, if the husband appears to be righteous, then he must be shielded [his faults concealed], and his wife should not be informed.