I can see the transaction on his address on blockchain.info:

https://blockchain.info/address/1MergNkpQKagk7cwEUb9MTk4qLy2TbtQfN

It shows his address (1MergNkpQKagk7cwEUb9MTk4qLy2TbtQfN) receiving the 4.23 BTC you sent. Either your friend is lying or he might have lost his wallet.dat file or something?