'English' is the nationality and England' is the country, so England isn't interchangeable with 'the British'.
England is one of the three countries that make up the island of Great Britain, The other two countries are Scotland and Wales. England is the biggest of the three countries by land mass, and by far the biggest in terms of population. The situation is further confused because the three countries of Great Britain + Northern Ireland form a sovereign state, the United Kingdom.
I've often heard foreigners say British/Britain when they actually mean English/England. You often hear people say that they like the 'British accent', when what they nearly always actually mean is they like a certain type of English accent.
People from the UK will tend to specify which part they are from, i.e. that they are English, Scottish, Welsh or Northern Irish. If someone asked me about my nationality, I would say that I am English even though I am British as well, because England is part of Great Britain.
I believe what your friend meant was that English is an adjective and can be applied to other things other than just people i.e. English food, English culture, English customs.
But 'the English' by itself refers to any person from England. Englishmen and Englishwomen are more specific terms, referring to the sex of the person from England