I can't even imagine how scary that was. Luckily for everyone involved (mostly your daughter), she didn't wake up and never knew she was alone.
I'm going to play devil's advocate... is he normally left home alone responsible for your daughter, or is she normally with you? If he's not used to having to take her everywhere with him, he may have honestly just forgotten that she was home - especially since she was sleeping so the house was quiet. (it's hard to forget a kid that's active and awake and under your feet!) Since she was sleeping, he may have forgotten that she did not go with you.
Has he apologized to you for forgetting to wake your daughter and bring her with him? Does he show remorse or regret? (Remember, he can regret the incident and be sorry for it even though he says it was an accident). If he has been stressed or not getting enough sleep, it may have honestly slipped his mind. Fathers bond with their children, but don't have that bond that we have as mothers. Though the child is a part of them, it's different when you carry that life within you. The fact that it was an accident doesn't lessen the severity and doesn't lessen his guilt, though.
If he said he's sorry and feels ashamed... I'd just let it go. Harping on it and being resentful is only going to breed more bad feelings - and let's face it - mistakes happen even with the most observant parents. Instead of focusing on the negative - focus on the positives of the experience. She (lukcily) wasn't hurt or scared and will will probably never forget to remember to take her with him again. Hanging it over his head won't change the past and may cause resentment and other negative feelings in the future.
Take a deep breath, let it go...and let go of the bad feelings about the situation. Everything is fine and it's in the past. Leave the past where it belongs - behind you - and move forward.