In the last couple updates the other girls started coming more into the front and Sandra is being thrown into the background, so I can only see two options, or Sandra accepts an open relationship, or she proposes that MC moves with her to wherever she's working now, he doesn't, and they break up.
I'm pretty sure Sandra will propose an open relationship with MC, she likes him too much and she will feel guilty for not fulfilling his needs. She doesn't want to do it but she also doesn't want to lose him, Lisa is always saying how Sandra tolerates a lot of things from him she would never do with anyone else. What will determine if she breaks with MC or if you get an open relationship will be if you were nice with her or not.
Right now it's basically impossible for the MC to be really faithful to Sandra if she will only show up in the weekends (not counting him fooling around with Haley), content would be very limited, Sandra will probably be swamped in work even in the weekends, so it will accelerate her decision, either with the break up or the open relationship.