I do hope that, at the very least, you will not make him pay the "locked in" price for a delay that is your fault. There are many threads on bitcointalk.org suggesting that these delays only happen when the BTC price is very volatile, and that once they are resolved, the actions taken by Coinbase (either cancellation of the order or delivery at a locked in price) always inure to the financial benefit of Coinbase.

I am certain that the Coinbase TOS allow significant discretion as to the timing and cancellation of orders. However, systematically delaying orders and then cancelling or forcing people to buy based upon which will be most profitable for Coinbase, which seems to be occurring, could certainly be construed as fraud.