(guilty) of adultery

unfaithful

(an instance of) voluntary sexual intercourse of a married person with a person to whom he or she is not married

extramarital affair

the act of causing a person to accept as true or genuine something that is false

practice of misleading

not honest

lying, untruthful

not loyal (to a person, one's country etc)

unfaithful

outside the regular course of academic work or studies

unfaithful

disloyal

disloyal

(of a person) often changing one's feelings about another person

vacillating, blowing hot and cold

(an act of) disloyalty or being unfaithful, especially to a wife or husband

disloyalty to an obligation

of lie, lie

dishonest

deceitful; faithless

treacherous

false or disloyal (to a friend, cause etc); dangerous, severe

dishonest, disloyal

the act, result, of being treacherous

disloyalty, dishonesty

deceitful(ly)

deceitful

not ethical

dishonest, immoral

not true (to one's duty, a promise etc); committing adultery

disloyal, adulterous

not patriotic

disloyal

without moral principles; dishonest

corrupt

not trustworthy

not dependable, unfaithful