He said "I imported the private key of B" but he might meant that he created it there. And then he said "The address is mine, I create it from the wallet!!" which probably meant that he used bitcoin core. I think that either one of them.
Edit: he generated those private keys using the blockchain.info web wallet.
So the moral is, don't reuse your old addresses, especially with different clients. Otherwise, you will get bitten if one of the client you tried is buggy. If possible, use a fresh address for every transaction.