… if your relationship (with that married person) is later found to have been the primary contributing factor behind ruining their marriage. And “costly” in that you might have to pay money to his/her ex-spouse (e.g., 20 mil won, jointly with your lover).
As you know, adultery was decriminalized in 2015. This means: adultery is no longer a criminal matter. This does not mean: you can’t be sued for having an affair (or being part of one).
1) If their marriage was already in shambles before you started seeing him/her, you will not be found liable. Causation is key.
2) No direct evidence of having had sex is necessary/required. Circumstantial evidence such as text messages can/will suffice.
In Korea, secretly recording a conversation you are a part of is perfectly legal. But if you secretly record conversations between your spouse and his/her lover, you can face criminal punishment.