Definitely call the SPCA! They will rescue and "rehab" the rat, plus give him complete vet care if needed. Your landlord ought to call, too, since he knows about it. That was a terrible thing those people did. If the rat had stayed loose and wandered off to go wild, he would not have survived at all. Those pet rats are bred in such a way that most of their innate survival instincts are no longer working.
Yes, they will eat anything. And yes, you did a really kind thing for the poor little guy.