if the photographer begged for his chance to take the pics. it's his business. if he is a learner, might be better to just take some shots in the middle of the day rather than waking up so early. do it without disturbing the main photographer's job.
that being said, assuming he is beginner, maybe the couple really was kind to allow him to turn up as the photographer. the couple also take risk mah... (if he is the only photographer).
For any normal couple's wedding, they really should engaged a full time pro to do the job.
you must also see the side where he spoils the market of the hundreds of AD photographers out there. those guys do a good job for a living leh... whereas your friend use the couple as his free models.