The software says my student cheated using AI. They say they're innocent

I rather like the French approach on this. If an assignment counts towards the course marks and is suspect, then the student is required to repeat the assignment supervised.

(This told to me by an English friend working as an English teacher in France.)

There is currently no reliable automatic way to know whether your student used ChatGPT. Best you can do is to manually check the structure and vocabulary of the work, but even that is not obvious with good prompting