Keylooggers used to catch a cheating spouse?

my frrend wants to know since her spouse has a keyloger aganst her can it be used in court? she knew nothing of it until just last week. is it illegal to use it against her since she wasnt warned about it? can a divorce be based on two peoples words being exchanged over email? how does it work exactly anyonnee know from axperiense or sometheng?

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