How to handle this oops

[umpharp]

ASA Jo ball

1 out, R1 on 3rd.

I'm on the bases. PU is a newer guy in his 2nd year. Overall a pretty good umpire, but still green.

Ground ball to short who throws to 1st for the routine 2nd out. As she throws, the runner from 3rd takes off for home. The 1st baseman throws home for the bang bang play at the plate and........The PU is starting to walk away from the play towards 3rd and isn't watching!

There is nobody to make a call. I call time. By this time, the PU realizes his mistake. We get together and he tells me that he thought the out at first was the 3rd out. I asked him if he saw the play at home and he said that he didn't see it very well.
I watched the play from my position and it looked like she was tagged out, but she easily could have been under the tag. There was no way for me to tell from where I was.

What would you do?

I'll wait for some responses before I tell you the rest of the story.