Jung-ho, the ostensible best friend, reveals his manipulative side in Episode 3. He meets Do-joon at a café and lies through his teeth, saying the tie was left there when he helped Sun-hwa fix a leaky faucet. Do-joon wants to believe him, but the audience sees the truth: later that night, Jung-ho sneaks into the house again.
If you are referring to a different "Mother" drama that actually has episodes: Mother (2018) A highly acclaimed suspense drama starring Lee Bo-young. Episode 3 Summary: Young Mother Korean Drama Ep 3 Eng Sub
“I came for the taboo. I stayed for the tragedy. Episode 3 broke me.” the ostensible best friend