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When we use an -ing verb as a noun. the preceding noun or pronoun must be possessive.
We use the double genitive (of + name + 's, his, mine etc) when we want to emphasise the person who possesses rather than the thing which he possesses. A friend of his is simply another way of saying one of his friends.
Where or not is implied, use whether, not if. Unlike whether, if does not expect a Yes or No reply: I shall speak to him if he comes.