It's hard to say, perhaps the other child is a brat with no sportsmanship to her at all, and perhaps she's just a green kid that hasn't got a clue what she's doing when she gets out in the ring.
In years of showing saddle horses there was only ever once when I was really cut off in a class; I was showing a stallion & a gal on a mare passed & then cut right in front under my horse's nose. Being a well trained, well mannered stallion he just sort of took a close look as he arched his neck & sucked back. I put leg on him & swung him off the rail and away from the mare's tail. If the other gal's intention was to wreck my ride then she was disappointed, because I won the class. At the time I was certain this gal had done this deliberately--she was a well experienced show person & should have known better than to do what she did--it wasn't a mistake I could see her making. Now looking back I do have to question whether it was a deliberate act of sabotage, or if she were just a little more stupid than I gave her credit for, and she didn't have a clue about what she was doing in that moment. I might have had something similar happen other times, but I was usually watching for such things & made sure I couldn't get boxed in and cut off.
I once sat & watched a local open show. One lady was riding around in a pleasure class, cutting off rider after rider. I really don't think it was deliberate, she was just cutting her corners & staying off the rail & was so absorbed in herself she just wasn't watching out for anyone else. I did see the results though--in the next class the other riders were definitely playing get even with her. Several of them were suddenly out to cut that woman off, and they did a fantastic job of it too!! And yes, I knew they were doing it deliberately--you could see it in their faces
and besides--when several riders who were previously riding very politely suddenly start cutting off one particular rider time after time, you know they are doing it on purpose!
But I'm going off course here....to get back to this topic, I agree with those who said to say nothing to the parents of this child. It's not likely to help, no matter if the kid is doing it on purpose or totally without knowing just what she's doing. The important thing is to teach your child how to deal with this if she encounters it again--if it's a decent sized ring and there aren't a lot of horses in the ring it should be possible to keep away from this girl.