Umpire Obstruction

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I had a play the other night where my partner obstructed the runner. I can't find anything in the rule book, so I'll ask here.
Situation: Nobody on base, batter hits a fly ball, bounces away from the fielder. BU does a lazy button hook into the infield while watching the ball. I watch the batter-runner touch 1st, heading for 2nd. I can see what's going to happen. The batter-runner is watching fielder chase the ball, knows she can make 2nd. As she turns her head to 2nd, my BU is right there, she runs up his back. After regaining her balance, she continues to 2nd, and is tossed out on a close play. BU calls time, comes and tells me, "I was in the way". I tell him, "Yes you were, do you have out or safe at 2nd?" He say's out, and signals the out. Coach is headed to the field, and I explain to him that during the play, umpires are just like dirt. I can't award her 2nd, and I can't send her back to 1st. He accepts it.
As soon as I get home, I'm in the book. I can only find Umpires Interference, but nothing that covers this situation. My question is, did I handle this correctly? What should I have done?