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There doesn't seem to be any way to reach the authenticators. And if tracking shows delivery to the authenticators, then they are currently the ones with the liability.
The holidays could have lengthened processing time by at least a week. If the item is seriously overdue (like a week or more), the buyer can open an "item not received" case. The buyer should keep in mind that they have 30 days beyond the delivery date shown in Purchase History to open a "not received" claim, so there is some time available to investigate before a claim is necessary (particularly if they want the item and not a refund).
When the claim is opened, it should be between the buyer and the authenticator, but if you become a party to the case, then add tracking and state the item is at the authenticator. After three days, either buyer or seller should be able to ask eBay to step in and deal with this non-standard situation. The goal here is to get a human agent involved and not let the bots close the case.
Otherwise, if you want to approach it head on, you can contact customer support and ask whether they have the ability to send a request to the authenticators to look for lost or stalled items.
Scroll to the bottom of this help page, click the "Contact us" option, and type agent into the chat that opens.
https://www.ebay.com/help/selling/selling-tools/ebay-authenticity-guarantee?id=4644
You may have to type "authenticity guarantee" and may need to type "agent" again to see phone options.
