M.,
I see a lot of good information here to mull over. I would START with how your friend and her husband treat him. Just like several other posts said, see if it's the environment first. If that fails, go the medical route - food allergies, sensory issues, etc.
We have gone through many "phases" with our kids. The hitting/pushing/biting phase was at the 3 yr. age. Sometimes it seemed like it was due to frustration or tireness other times it seemed unwarranted. Remember that 3 yr olds don't always have the best communication skills. They don't know how to always say, "I'm tired." Or "he just stole my toy". Instead they melt down or fight instead of "using their words". I know that was a big key for us - USE YOUR WORDS.
Also, if Mom and Dad are arguing, or if the child is yelled at/spanked then he might think that's how he should/can act. There are several disciplining books out there - but I'll be the first to tell you, you can read all you want, the results are in changing YOUR behavior. (I had a "friend" who read all the books, but never stuck with anything long enough to get results.)
We use "time outs" in our house. Also, "Keep your hands to yourself." And "Use your words." Mom and Dad don't hit, so you don't get to either. I think this age can be especially difficult for kids who don't know how to talk well enough to express themselves.
Hopefully by changing their behavior, and encouraging their son to use his words (instead of his hands), that might be a good first step.
Also, it's common for kids at this age to "test" others - to get reactions (like pulling a siblings hair to make her cry) or defiant to parents/teachers to see what's going to happen. I hope your friend can nip it in the bud NOW, because it will only get worse.
Best of luck.