How can I safely cut this pipe with an angle grinder?
Remember, cast iron is brittle.  A chain-breaker does not actually cut it, it cracks and separates it.  You can use the same principle: use the angle grinder make a groove to weaken material around the perimeter of the material  right where you want it to break.  Then allow the grinding wheel to penetrate all the way through at only a single spot (a 1/4" or so).  Then use a regular cold chisel to finish the job:  Drive the chisel in at a tangent, following the groove you just cut.  The wedge shape of the chisel will lift the cup up and away from the pipe, and like cutting glass, it'll snap right where you weakened it.

I've done it many times, and it only takes a few minutes.