Just ask for screen shots of his account,find it hard to believe there would be no record in both accounts. If it did vanish it would still at worst show in his records.
You hint at trustworthiness and debt and I think you already know the answer before posting it. Ask for a screen shot from him and see what happens.
But it was in there on march 22nd, now its gone. I didnt transfer it anywhere, and its like it was never there
Do you still remember if the transaction has any confirmations? If there were no confirmations, the fee might be too low and nodes have dropped them out of the mempool.
If you still remember the origin address, check if the balance is still there or another transaction replaced your transaction.