In the following sentence why do we have a comparative and not a superlative? What's the difference between them, please?
"He had first picked her up in London, at one of his richer friendís."
Why not "RICHEST friend's"?
Richest as in best, one of a kind. Note the structure here:
 Some of his friends are rich and some of his friends are very rich. His sister was staying with one of his richer friends, with Max, at Max's house.
 All of his friends are very rich, but one is richer than all the rest. His sister was staying with his richest friend, with Max, at Max's house.
Hope that helps.