My god there is a lot of wrong in this thread.
The best way to think about it is what would happen if they were to close the account immediately after the payment.
Account now has £400 in it for simplicity sake. £200 is the OP's and £200 is the OP's friend's.
Step 1: OP pays in £600
Step 2: OP Friend pays in £1,000
Account now has £2,000 in it. They go down to the bank and close the account and withdraw all the cash. They split it 50/50 as per the agreement so each gets £1,000 out. The OP has therefore only gained £200 in this deal (the £1,000 he gets when closing the account less the £200 he had in to start and less the £600 he has just paid in).
The OP's friend however has only lost £200 too (the £1,000 he gets when closing the account less the £200 he had in to start and less the £1,000 he has just put in).
For it to work, the OP must pay in £1,400 at the same time as the OP's friend paying in £600 in order to maintain the £2,000 injection of cash.
The OP's friend will never get this point across though as the OP's friend will think "i'm paying £800 more than you!!!!"