Continuity mistake: While Sandy is singing and they all have their arms around each other, from the front, you see Alex is next to Zach, but from the back, Marissa is standing next to Alex and not Zach.
Continuity mistake: Sandy's hands change from being on the microphone and being in the air between shots while he's singing.
Answer: He means that if Alex has to go so far as to forbid Ryan from seeing Marissa in order to maintain her relationship with her, their relationship obviously isn't working.