While I would say give it back I can see why he may have given you the cheque.
You were best mates for 10 years! Had you not been friends maybe he would not have been such a success. He must have had friends since 1996 but the friends you have at certain times of your life can influence somtimes what seems like small changes that change the direction of your life.
I realise you don't wish to accept the cheque but if I were him I would give £10,000, maybe more to friends I have know through out my life.
I guess he gave you the cheque to thank you for being such a good mate back when you had no money. He may feel guilty about how much he earns and it's his way of helping others BUT people he knows rather than just unknown strangers.
I know people with money who wouldn't give away £100, if you give the money back it would be just another £10,000 in his bank balance.
He hopes you put the money to better use than he would. With the money he's got now how can he be sure who are REAL friends are? Should you accept the cheque I would buy him a small gift but try to think of something that has sentimental value.
Whatever ever you decide I wish you both the best for the future.