Woah! She resembles his mum and Bum a lot! (There also is nothing more annoying than badly...

It is more likely that Sangwoo is just having a delusion, and is projecting the image of his mum onto this woman, rather than her literally having and resemblance to her. He did the same to Bum earlier in this series. Sangwoo's flashbacks to his mum are increasing, and his decline is becoming more apparent. His episode on the ski slop, getting distressed and panicked over the mother and child scenario was just another example.