It may just be the way he handles medications such as Ambien. My mothers partner doesn't handle narcotics such as Ambien or Vicodin well at all. On Christmas day last year we were all hanging out and she "blew her back out". Sometime that night she got up and took some kind of pain medication. I am not sure what it was. The entire next day she was a complete crazy person. She was very argumentative, yelling, screaming, acting in a way I've never seen. She was walking into traffic, trying to break out of the hotel room we were all staying in, etc. It wasn't pretty and somewhat ruined a nice Christmas. The next day, she had no idea what happened. She remembered none of it. She didn't remember anything past going to sleep Christmas Night. She doesn't remember what she took or when she took it.
Some people react differently to medication. I would recommend talking to his doctor about it, because it may not just be the Ambien.
I edited this to add:
Yes, we were drinking that night, so alcohol may be a factor in this. I personally can drink and take Ambien. I can also drink and take Xanax and not be affected at all. Not tired or anything (not that I would EVER drive at that point!)... Unlike my grandma, who gets completely knocked out with 1/2 a xanax and no booze. So every person reacts differently to meds.