You can set up a long term (if repayment is more than a year away) liability/loan account for the friend.  "Deposit" the amount into a clearing account (create a bank-type account if you don't already have one), using that loan account under the "from account" column.  Then "pay" the bill, using that clearing account as the bank source.  The balance in the clearing account should be zero, and the liability account will reflect the loan from your friend until it's repaid.