People tend to play a LOT more aggressively HU. I've seen all kinds of wtf? plays. Heads-up isn't so much about the cards, as it is playing the player. But--obviously this guy is horrible if he's trying to steal at the 20/40 blind level by shoving with 10/5. It just sucks that you had him so dominated and he lucked sacked on you. Happens all the time though...as a poker player you have to learn to just roll with it.
But, if you really think there was something shady going on (I really doubt it, but hey), send the info to the site's support so they can investigate.