Fave football player has a partial torn rotator cup, is that something that can tear more?

Yes: Many rotator cuff tears can increase in size. However, many partial tears can remain the same size indefinitely. Many throwing athletes have partial tears that are physiologic (or normal for their sport). Many pitchers, for example, develop small partial tears that become part of the way their shoulders work and do not need to be repaired.

Unstable Shoulder?: Typically, the concern in a high level athlete as you describe is that the rotator cuff tear is a result of some other primary cause such as shoulder instability. With persistent instability, more and more joint damage and tissue injury can result. Blog article on this topic: http://www.Theshouldercenter.Com/shoulderpain/2012/shoulder-surgery/playing-can-worsen-already-damaged-shoulders/.