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It's very possible that your friend is having a delayed reaction to the dissolution of her engagement, especially if she didn't react strongly at the time. Perhaps she suppressed her emotions then, but for whatever psychological reason is allowing them to bubble up now, triggered by the latest breakup. Encourage her to enter counseling, so that she can work through her feelings about both relationships and about herself. Support her in whatever way she wants, whether that's listening to her talk or helping her escape from the pain once in a while. And do watch out for her -- even if that means occasionally "checking up on" her. It sounds like she may need some serious nurturing. Best wishes to both of you...

If her love and attachment to the most recent bf was very strong, it doesn't matter that they only went out four months. I've been devastated by the breakup of a relationship of the same length. I would agree with the others who suggested therapy/counseling. Also just be there for her, try to see what her biggest concern is. Is it the general loss of love? Does she think she'll never find it again? Does she not understand why they broke up? Does she regret doing something that she thinks caused the breakup? Be supportive and reassure her that all things pass, and reasssure her as best you can about whatever is bothering her the most...

get her out of the house. keep her busy. talk to her lots. and when she does need to cry...let her cry on your shoulder...i know its really hard to deal with the after math. and kinda puts a downer on your day..but just be there for her, she will need you..

I've been in this situation (hers and yours.) As far as I know there's not a whole lot you can do except be a supportive and distracting friend. She's going to have to come to terms with her emotions on her own. When I felt the way she did I only started feeling better when I found another guy; possibly the man who just left her had made her feel better about her fiance leaving her, which is why she is reacting so strongly...
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Make sure she eats and doesn't hurt herself, and just try to be the best friend you can be...

How long after breaking off her engagement did she wait before this guy?..Maybe she moved quick enough and was in a relationship soon enough after that she never had time to be sad...Now, she's all alone and it's finally hitting her...