I believe so.
Go to your private messages and check 'tracking'. If the person is still in the 'unread' list, they obviously still haven't opened the PM. If this is the case, go into your sent folder and check the box next to the sent message. Scroll down to the bottom of the page and click delete. This should delete the message. After doing that, I'd go back into 'tracking' and see if the message shows up anywhere. If it's disappeared from the 'unread' list (and still had never made it to the 'read' list), I'd say you're good to go.
Good luck.