I went to primary school in a London borough in which there are no Grammar schools, and many of the Secondary schools at the time were not very good in terms of results or behaviour.
I was in the top ability groups in each subject at primary school and has really high aspirations, and would achieve top marks in any tests.
A neighbouring borough have some Grammar schools, and their non-selective schools were are far better than those in the borough in which I lived.
I didn't receive any coaching or tuition for the 11+, but my parents bought me 11+ practice books from WH Smith's, and they would help me to go through these with me. They never placed any pressure on me, it was me who applied pressure on myself. They told me that if I didn't pass it wasn't the end of the world, and I would still do well regardless of what school I attended.
I ended up with 4 marks short of the requirement for entry into a Grammar school. I was really disappointed with myself, and I cried. My parents reassured me that I would be fine, and that I stood a better chance of entry into a non-selective school in that borough, as at the time they wouldn't consider people from outside the borough who hadn't taken the 11+, except for in exceptional circumstances.
I attended a Secondary school in that borough, and I was in the top sets for all of my subjects. I felt I was really able to flourish, and I was recognised as an over-achiever by my Teachers. I went on to achieve the best GCSE grades in my year group, and again at A Level. I was a very quietly confident and assertive teenager, mature beyond my years (according to my school reports!)
I really do not think I would have done so well if I had achieved the extra points needed for a Grammar school, as I would have just got in, and would've been in the lower sets. I can't imagine that would've done much for my confidence.
Perhaps taking the 11+ makes it easier or possible for your friend's child to enter a non-selective school in a particular borough, as with me?
That is the only situation in which I would advise someone to take the 11+ if they couldn't access tuition or were of average intelligence, otherwise I don't really see the point and it could be quite damaging to the child in my opinion.