What do you mean leave? Like leave Oil? She didn't leave Oil, she fell off the boat, got rescued by human traffickers and lost her memory. She gained her memory on the day oil was getting married and that's when she decided to come back and ask for her fiancee back.
If I was marie, I would just let him go too. It like what her sister said, she can fall in love with someone new. Marie's only reason she's not letting go is she loves Oil, which is understanding bc it's not like she snatched oil away from pang. And just bc she came second, doesn't mean she's at fault and must give up her husband. Marie's dad is involved in this situation too much, I think. If it wasn't for him, marie was willingly to sign the divorce paper right then and there.