A friend of mine married a girl who is much (young) than him.

The correct answer is "younger". When comparing two things using adjectives, we often use the comparative form of the adjective. In this case, since we are comparing the age of the girl to the age of the friend, we use the comparative form of "young", which is "younger".

So, the correct sentence is: A friend of mine married a girl who is much younger than him.