Can external thesis examiner object to inclusion of so many unwanted co authors in published papers? My friend is working on material science and he published 3 papers in very good SCI index journals. In all papers he is first author and second is his supervisor and rest are co-authors like incharge of lab where test were conducted, teacher of his supervisor who is working in other university, some other scientist from USA who suggested the problem to his supervisor but was not involved in work in any other way and last co-author is student of his supervisor from same semester working on slightly different problem. In each of three papers the order and number of authors is same.

Now he is afraid of external examiner of his PhD thesis who may according to him might raise objection on inclusion of so many coauthors. All the papers were communicated by his supervisor so there was no chance for him to decide of his own. I told him that, this is a usual practice and nothing to worry about. Am I right ?
Now he is afraid of external examiner of his PhD thesis who may according to him might raise objection on inclusion of so many coauthors. 


Six authors listed on a scientific paper in a lab-experimental discipline in not a very high number. I don't know what the specific norms are for materials' science, but that's not uncommon.

Also, there could be 100 co-authors - that doesn't matter, as long as they don't argue about the individual contributions each one has made. Your friend has had an opportunity to explain what he personally worked on, in the context of producing these papers and more generally: That opportunity is the thesis, which ties everything together.

If your friend is in a thesis-less track (i.e. submitting articles only to be considered for awarding a Ph.D.) s/he may want to consult his/her advisor regarding how to transmit additional material - e.g. a letter - to the examiners. In fact, consulting the advisor regarding this concern is a good idea regardless of the measure to be taken.


  this is a usual practice and nothing to worry about. Am I right ?


Probably - but your friend's advisor will be able to confirm that.