Can external thesis examiner object to inclusion of so many unwanted co authors in published papers?
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.