DEAR MISS MANNERS: Is it rude to give advice to someone who did not ask for advice? Under what circumstances is unsolicited advice polite?
GENTLE READER: Unsolicited advice may be given discreetly if it will save the recipient from imminent embarrassment.
Miss Manners used to cite the example of one lady taking another lady aside to warn her that her underwear is showing, but she realizes this may no longer meet her stated precondition.
Beyond this, etiquette limits the privilege of giving unsolicited advice to teachers, mentors and parents -- and only when acting in that capacity. Be warned that etiquette having no objection to such proffers does not guarantee a welcoming reaction from the recipient.