Not everyone locks the door. Maybe he was awake in bed or in a dark room and got up when he heard someone at the door.

Her actions are negligent at a minimum, but haven't homeowners "gotten off" when they shoot their roommates, spouses, etc. and say "I didn't expect them to be home and thought it was an intruder"

I don't know what she could be lying about, there's just no evidence apparently that she had any reason to kill him.

Let's say she was asleep in her own dark apartment, leaves light off, he accidentally walks into work apartment, she sees a figure and immediately shoots and kills him. Is that manslaughter?

Does is matter whether she mistakenly walks into his place, or he mistakenly walks into hers, before she starts shooting at anything that moves?

Interesting case. I imagine she'll say she didn't even know he was black.