In England and Wales a testator can bequeath their estate to whoever they like and there is absolutely no obligation to give anything to relatives. (My own will doesn't leave a penny to any of my family).
However the Inheritance (Provision for Family and Dependants) Act 1975 gives certain people (who would certainly include your friend's son) the right to apply for a court order varying the terms of the will. But such orders aren't made just because a person feels disgruntled by being left out of the will. The son would have to show that the will failed to make 'reasonable financial provision' for him.
The law defines 'reasonable financial provision' as "such financial provision as it would be reasonable
in all the circumstances of the case for the applicant to receive for his maintenance"
When examining those circumstances, a court must have regard to all of the provisions listed here:
http://www.legislation.gov.uk/ukpga/1975/63/section/3
If the son is currently living with his mother (or has very recently done so), and is supported by her, then he may well have a valid claim for 'reasonable financial provision'. However if she has clearly disowned him and he has not recently been financially supported by her, any claim would be far weaker.
It's impossible to write a will which will guarantee that certain close relatives won't benefit from the estate. The Inheritance (Provision for Family and Dependants) Act 1975 was enacted specifically to stop that happening where it would genuinely disadvantage people who had a reasonable right to expect to benefit from the estate. However (despite how the media might portray them) judges aren't fools and they won't vary the terms of wills unless there's a clearly compelling reason for them to do so.
Watch out for a possible reply from Barmaid on this thread. She's a barrister specialising in such cases, and so will know far more than most other people on here about such matters.
Chris