So you send
BTC from MultiBit wallet to blockchain wallet, that blockchain wallet is not your wallet since you write :
"i sent them to somebody and he has proved he never received ". Is this blockchain wallet your wallet or wallet of some other person? If this wallet belongs to you, to log in you need ID and password, if this is not case I do not see any reason for trying get back that coins.
You also say that you use Multibit wallet, is it MultiBit Classic ( no seed ), or maybe MultiBit HD ( with seed )? HD version of wallet is first time released in June 2015 ( version 0.1 ), so it is very possible that you are using Classic version.
In Classic version you only have option to export private keys ( there is no seed ), so if you have original wallet file you can try to get your private keys - in case you think that somehow you still have that
BTC in your possession. If you get private keys it is easy to import them to any wallet.