Can a person possibly become manic depressive if they watched someone die?

Not likely: That would be a very disturbing thing and might clearly warrant help, but really not enough to "cause bipolar \disorder" though psot trauymatic stress disorder might be possible to be stemming from such an event. Sounds like a professional evaluation is indicated.

Trigger: There needs to be a predisposition for manic depression, for a person to express the disorder. However, a major loss could precipitate a depressive or hypomanic/ manic episode in someone who has bipolar disorder or who already has the predisposition. It would not cause manic depressive but could trigger symptoms.

Not likely: It is more likely the person would likely be deeply traumatized and depressed from such an even, but not manic depressive.