Tell your friend to spend more time with him - playing with him, taking him for walks, letting him run about outside - supervised by an adult, going to feed the ducks at the park, play football with him, teach him to catch a ball, take him swimming, anything else that she and her young child will enjoy.
Keep him amused - let him do finger painting - on paper and not on the walls - drawing, teach him the alphabet and how to write his name, baking. Kids, even at his age love to break an egg into the bowl and make cakes or gingerbread men. He needs to have his mind occupied in learning things. Talk to him lots, read him suitable stories before he goes to bed.
He can help his mum load the clothes into the washing machine and press the button to switch it on. Kids love to help as well as to play.
Make sure that he does not stay up late. At his age I would put him to bed at 7.30 (go earlier if reading a book) at the latest and he should also have a short nap in the middle of the day. Children can be very naughty when they are tired or when they are coming down with an illness. As he is always like this, the parents need to take control and structure his day.
I feel the reason for the head banging is due to frustration because he is not able to express himself adequately just yet and tell his parents what is bothering him or say what he wants.
Ensure that he is NOT watching any unsuitable TV for his age.
Do not give him too many sweets, sugary drinks or fruit juices. Cut down and only give as a treat, cakes, biscuits, chocolates. Make sure he gets a healthy and well balanced diet with any fruit that he likes.
Sometimes, you just need to peel an apple or place some fruit in front of a child and they will then eat it, rather than placing sweets, crisps and chocolates in front of them.
He needs discipline (not shouting or hitting). Just firmness and a structured day and sleep pattern.
The parents also need to give him lots of hugs. He does need to be put right if he is doing anything wrong straight away, but don't keep going over it for days on end. Young children have a very short memory span.
Once he is in a good routine and is getting lots of parental attention, he will settle down.
If all this fails, an appointment with a doctor to get the child examined for any developmental issues should be considered.