In the Modifiers Strategy Chapter, under Noun Modifiers with Relative Pronouns subheading, it is mentioned that ''..whom is used as the object of the verb or of a preposition." and an example below it is "The security guard whom we met was nice. "
Below the example its written that - "That or whom can be dropped when the modified noun is the object of the modifying clause"
So I understand from this that the modified sentence - "The security guard we met was nice." is also correct.
Please let me know if this is the right interpretation. Thanks in advance.