Read what Fanjo said and ignore the pitchfork wielders.
To me it sounds like he tried to be helpful - making her sit on the toilet when she didn't want to is not helpful, so you should set boundaries there, however, I don't think it's neccessarily bad that he wants to help, and that potty training should be down to you and you alone - when a child is learning, it needs adults to help. When mummy is busy otherwise, it's a good thing that another adult helps instead.
In the potty training of my daughter there were many people involved - me, my husband, my inlaws, the nursery nurses, her friends' parents etc. Never occurred to me that it should be left to me alone.
He's done it wrong, granted, and it;s an arse to undo the damage, so you need to talk to him about that. But in general it's not wrong that he wanted to help.
Also don't quite get what the issue is with him sitting between you and your daughter?