If a person has a PhD in area X, and suddenly, giving comments to a candidate that is in a completely different field, Y, such as; "this is not Y!"; is the person allowed/qualified to do that?

I was this in this room where this panel member said the exact same thing as the above and that panel member is not even in the field! How so that the person can give such judgement?
(The person's job at the time was to check the writing aspect of the thesis)

Any opinions?

In an ideal world it wouldn't happen, but it does all the time, especially with people who have a little knowledge but no real understanding of a subject. It's happened to me a few times. You just have to ignore it where possible and defend your position when necessary - which usually isn't too difficult given that these hecklers are not often well informed.

http://www.postgraduateforum.com/thread-23251/ is this the same incident you were worried about in this thread? I can't imagine a scenario in the UK where an examiner would be appointed only to check the writing aspect of the thesis - what country is this in?