How would you be able to do this instantly where as to not cut in to our profits? Alt Coins usually withdraw and deposit pretty fast so you would need to offer something better then the 5-10min window.I would be wiling to pay a fee of course if this is possible since just yesterday I had to do a transfer from mintpal to cryptsy and it took almost 12mins for the deposit to confirm so I could trade.
I think the idea here (at least the way I envision it) would be for him to have coins on two (for the example) exchanges.
Exchange 1: 100 BTC
Exchange 2: 100 BTC
You then tell him you need to transfer 50 BTC from exchange 1 to 2. He would send you (through the p2p/user transfers) the 50 BTC on exchange 2 and you would send 50 BTC + fee to him on exchange 1.
That deal would be done. He'd then equal things out on his own by sending the 50 BTC back from 1 to 2 on his own account, preparing for the next customer.