Honestly, if you take her power as working properly, she probably could rely on it, since she likely has trained to tell the difference between a heart during exercise and in pain, and one that is lying.
It's just a simple matter of cheating and using a false heartbeat then, a cheap and simple trick if one can pull off the acting to go behind it.
The point is, the best players in this world just cheat, and cheat like hell. Special training is just as cheating as rigging a heartbeat detector.