I am a UK citizen and a friend of mine plans to visit me in the UK during the summer. He is planning to stay one month. He is a recent graduate and his parent are rewarding him with a holiday to London. The friend would be staying with me during his trip. 

To support his visa application, would there be any documents I would need to provide? Would I need to be earning a certain amount of money? 

Since his parents are paying for the trip, their bank statement instead of his would be fine right? And how much would they need in their bank account?