So the other girl having the same answers is the only reason why your DD thinks she cheated - it wasn't clear to her at the time? What kind of table arrangement did they have? A competition should always have people sitting far enough apart that students can't copy one another's answers without it being obvious; perhaps that wasn't followed in this case?
I don't think you can "do" anything about it in this instance, really; cheating needs to be detected while it's going on. Pointing our the coincidence of answers at this point is going to incriminate your own DD just as much as the other girl (and particularly as they're friends, they'd both come under suspicion).
Also, it's difficult to pick truly random numbers in the context of a maths question; it's not out of the question that, in fact, your DD's choice was subconsciously influenced by something in the question or around, and the other girl might have been similarly influenced - seems unlikely they'd come up with exactly the same answers [and is there a possibility that your DD is exaggerating and only e.g. one of her random nos was the same?] , but not impossible. (If the numbers in question truly were random 3-digit integers, I'd be inclined to ask your DD what she thought the probability of just happening to be correct was, and whether she thought that was worth the time it took to write the numbers down, when compared to the alternative use of that second or two to doublecheck a non-random answer!!)
I would suggest pointing out to your DD that it's in everyone's interests to make sure copying is impossible, and making sure she realises that if her answers are copied she too is going to come under suspicion. Another time, she might request that tables be further apart if she thinks participants seeing one another's answers is going to be inevitable (it's offputting in a test to have to be thinking about keeping your eyes away from other people's work, so she could phrase this as wanting to be sure she didn't accidentally see anyone else's answers). Or she could perhaps choose her own position with care, and/or guard her answers. Unless they were actually sitting at the same table (which would be crazy in an externally run competition!) it shouldn't be hard for her to keep her answers from being copied without being obvious about it.
Other than that, I'd just treat it as a learning experience. Nothing can be proved at this point, and casting suspicion that can't be proved is just poisonous.
