@Mor - I would go and see a doctor, but the chances are that it's not going to be a big deal. It sounds very serious to say someone is vomiting blood, but an irritated stomach lining will lead to that and it can be caused by eating the wrong thing, or drinking too much.
So I wouldn't completely panic unless the person is showing a lot of other dire symptoms. But don't leave it either, since it can be a sign of something worse. And most of those worse things are the kind of thing that you really need to get seen to right away.
We like to think that the stomach is very strong, but if it is treated the wrong way it can go downhill very quickly.