off her guard vs off one's guard

off her guard and off one's guard both are adjectives.

Word	Noun	Adjective	Verb	Adverb
off her guard	No	Yes	No	No
off one's guard	No	Yes	No	No

As adjectives, off her guard and off one's guard are synonyms defined as:

Other synonyms of off her guard include napping, off-guard, off guard, off his guard, off your guard.

off her guard (adjective)	off one's guard (adjective)
not prepared or vigilant	not prepared or vigilant