Hi @Ariane473
It sounds like Support misunderstood and accidentally sent the link where hosts can request a review removal.
I've checked for you and I cannot click somewhere to remove a review that I've written. I've also searched the help articles, and that confirms that only way to remove a review that you've written, is to contact customer support (which he is having trouble doing).
If all else fails, you could eventually click the link they sent and (from your side), ask Airbnb to remove the review based on the fact that the guest agrees to the removal. It may be a struggle, but if they're willing to contact him to confirm, it should theoretically be possible.
(By the way, it doesn't make sense to me that the guest accidentally removed his own 5-star review. One cannot remove this yourself, and customer services would surely not have removed an all-positive review at his request?)