So, I'm a new DM, this is my first time. One of the players in the campaign really wants to have her character abandon the group during the night or even while they're at a city; and just go off by herself. I told her that she could do whatever she wants, but silently I'm freaking out; because i'm not at all sure on what to do. 

One of the reasons the character is leaving is that another character has completely smashed everything that is important to her in the campaign; and kinda screwed everything up that I had planned. However the main reason is because she just wants to leave; with no particular reason. Having two separate journeys going on at once is something I do not want to do for my first time.