What happens if someone overdoses in front of you and then dies are you partly responsible if you done nothing?

Yes you are partly to blame as technically you are assisting their suicide (if the overdose was intentional) and euthanasia is illegal in most countries in the world with only one or two exceptions. If you watch someone die and it wasnt intentional on their part it would probably be regarded as manslaughter as you did nothing to aid their survival. I hope this hasn't happened to you as its pretty horrible either way.

It is called gross negligence and amounts to willful or depraved indifference to human life. You can be charged and prosecuted for this.