@Tony1182 When Airbnb collects evidence for claims and disputes, it never transmits the original documentation to the other party. This alleged video is very unlikely to be an exception. I don't think there's anything you can do to force the guest to send you the video, so you're probably at a dead end there.
What you can do is edit your Host Response. It's nice that you've gotten mostly 5-star reviews in the past, but that's an irrelevant detail as it doesn't prove that you don't have a flea problem now. You do allow pets in the property, and clearly at least one of your other recent guests brought pets because their review specifically says "wouldn't recommend bringing animals." Based on these accounts, it sounds entirely plausible that a previous guest left you with a flea problem that only became noticeable in the most recent stay. Of course, this can happen despite a meticulous cleaning - especially when you have carpeted floors and upholstered furniture.
Guests do not feel encouraged when hosts respond to negative feedback by bragging about other guests' ratings. They want to know that you've taken the issue seriously and made the necessary steps to address it. It may turn out that there isn't a flea problem after all, but resting on your old laurels does not offer any assurance of that.