Go past the part where he falls in and when the Guide leaves you and Margret alone, decide to go over and see what's going on with the guide instead of staying with her.
I'm happy it worked out! For Margret's last CG, you need to spend your time with her, be understanding towards her, make sure she doesn't die, and only lose one person (Bem or Lu, but not both) on the first night. On the second night you need to help the guide when he's in trouble on the shore, then eventually, choose to stick behind with Margret and survive the second night with her.