Technically she saved his life twice.
Once by forcing the coin on him, (last weekend) and today by doing what goes against her nature.I completely agree with Riya though, shooting a target and shooting a human is way different, and Arjun forced her hand today.Her reaction is completely natural, she killed a person for the first time, but I bet you she won't hesitate anymore either.I have heard it say that after the first killing, cops get used to taking lives, and I believe Arjun's reasoning is also right, think about the reason for the shot, don't think about the target.His reasoning to Riya is also 100% correct, at some point she has to think like an officer that she wants to be, not like a typical girl.