this is so common, we could practically have a topic for 'children who are being potty trained and don't like pooing in the loo/potty!'
What worked for us was trying to take the anxiety out of the situation - we had a lot of laughs about how funny poos are, how lovely the toilet is etc. - I know it sounds like a bundle of laughs but it was the only thing that worked! I should also say that it did take a while to crack and every time he had a 'sore' poo (if he ever got slightly constipated or had a big poo) then we had to go back to square one again.
Has she got a special seat for the loo? Also found that ds liked the soft seat (the sort of squishy ones) more than the hard plastic ones.
There are books you can get - one I think is called 'Everyone poos' or something like that. I will look it up.