Dealing with bi-polar

My wife is diagnosed bi-polar. She has recently had difficulty controling her temper and sometimes becomes very angry over things that would not be considered to be worthy. For example, yelling at my pre-teen daughter and causing her to cry because she was having difficulty opening a new combination lock for school. How do I intervene and prevent things from getting to this point without making it worse for them both? After these episodes of anger cool down she feels extremely guilty and gets down on herself.