Right click the transaction in the transactions list and choose "Abandon transaction". Then he can resend the Bitcoin but this time include a much higher fee. In Bitcoin Core, you can set your fee by clicking the "Choose" button next to where it says "Transaction fee" on the send page. Then choose the option for "Recommended" and move the slider all the way to the right where it says "fast". Note that in order for the fee estimation to work Bitcoin Core must have been running for a while.