It depends on a number of factors. The nature of the relationship is one: incidents that occurred during the relationship, feelings of unfairness, etc., can greatly affect how someone handles a breakup.
Also, past experiences have a great deal to do with it, including family life. Maybe someone doesn’t have a very supportive family, and they feel that they need a boyfriend or girlfriend to fill that gap. Thus, when the relationship ends, they feel abandoned and can’t handle the thought of not having someone to support them anymore.
Likewise, if someone has had more relationship experience, I find it makes it easier to deal with breakups. They’ve been through it before, they know they can get through it and go on with their lives. Especially in a first relationship, it’s common to think that it will last forever, that their significant other is the only person for them. It just takes time and experience to deal with these things.
Hopefully your friend will learn to deal with the breakup. It takes time. I’ve heard somewhere that the time needed to get over a person is about ⅓–½ of the time they were together.