Who is "the man in Dickens"? The answer is that Chesterton is referring to the following passage from Charles Dickens's The Old Curiosity Shop, in which the question "is the old [man] friendly" is repeated twice by the character of Mr Swiveller:
‘Fred!’ cried Mr Swiveller, tapping his nose, ‘a word to the wise is sufficient for them—we may be good and happy without riches, Fred. Say not another syllable. I know my cue; smart is the word. Only one little whisper, Fred—is the old min friendly?’
‘Never you mind,’ replied his friend.
‘Right again, quite right,’ said Mr Swiveller, ‘caution is the word, and caution is the act.’ with that, he winked as if in preservation of some deep secret, and folding his arms and leaning back in his chair, looked up at the ceiling with profound gravity.
‘Fred,’ said Mr Swiveller stopping short, as if the idea had suddenly occurred to him, and speaking in the same audible whisper as before, ‘is the old min friendly?’
‘What does it matter?’ returned his friend peevishly.
‘No, but is he?’ said Dick.
‘Yes, of course. What do I care whether he is or not?’
Emboldened as it seemed by this reply to enter into a more general conversation, Mr Swiveller plainly laid himself out to captivate our attention.
-- Charles Dickens, The Old Curiosity Shop, Chapter 2
It's later referred to again by the same character:
‘Before I leave the gay and festive scene, and halls of dazzling light, sir,’ said Mr Swiveller, ‘I will with your permission, attempt a slight remark. I came here, sir, this day, under the impression that the old min was friendly.’
‘Proceed, sir,’ said Daniel Quilp; for the orator had made a sudden stop.
‘Inspired by this idea and the sentiments it awakened, sir, and feeling as a mutual friend that badgering, baiting, and bullying, was not the sort of thing calculated to expand the souls and promote the social harmony of the contending parties, I took upon myself to suggest a course which is the course to be adopted to the present occasion. Will you allow me to whisper half a syllable, sir?’
Without waiting for the permission he sought, Mr Swiveller stepped up to the dwarf, and leaning on his shoulder and stooping down to get at his ear, said in a voice which was perfectly audible to all present,
‘The watch-word to the old min is—fork.’
-- ibid, Chapter 3