Dear Laura,
The angry behavior per se is not that unusual for a child his age. Of course, parents should be setting reasonable limits on him - often such anger is in response to limit-setting. Children become overwhelmed with anger when their wishes are frustrated. They haven't yet learned to tolerate frustration, and this developmental task can be impeded if parents don't expose their children to reasonable types of frustration.
Relative to the little boy's speech, he may be displaying a delay in his acquisition of speech, or some problems with articulation. This difficulty perhaps should be assessed by a speech/language professional.
Poor kid. My 5 yr old did the same thing last year. It is partly frustration and partly extremely poor self esteem. Have your friend give him specific praise for good behavior and not go off the deep end about the bad. No yelling or spanking, just calm discussion about his behavior and what the correct behavior is. Lots of love and attention. The ratio of specific praise to correcting bad behavior should be 10 to 1 each day (hard to do but worth it). The only thing that will help is boosting their self esteem and teaching them skills to ease their frustration level.