His board, depending on when it left the factory, might have a late enough BIOS to work with that CPU. Another possibility is that AMD had some deal where they would send you a CPU that will allow you to update the BIOS. (I assume you would then have to return the temp CPU).
With the serial number from the motherboard you can call ASUS and they should know which BIOS it shipped with...
Workarounds
The following workarounds may be feasible for affected users, depending on their individual circumstances, and should be considered before applying the boot kit solution detailed below:
Update from Retailer
If the motherboard was purchased through a computer parts retailer, check with the retailer to see if they can facilitate the BIOS update at their location. There may be a charge for this, or it may be offered at no cost.
If you have access to a processor supported on the current installed BIOS, use it to perform the BIOS update, then swap out the processors when the update process is complete.
For a list of supported processors per BIOS version, please refer to the CPU Support List document available on the motherboard manufacturer's website. BIOS download and installation instructions are also found on their website.
If these workarounds are not feasible, please see the sections below for a solution.
Original Manufacturer Replacement
The Original Design Manufacturer (ODM) of the motherboard will support affected end users with an RMA exchange for a compatible motherboard, upon request. Affected end users are recommended to contact their local representative for the motherboard manufacturer to exchange their motherboard.
Short Term Processor Loan "Boot Kit"
AMD will offer affected and qualified users (as described below) a AMD AM4 A-series processor including thermal solution for short term use ("boot kit") to perform the BIOS update on their motherboard.
This short-term processor loan or boot kit is offered through AMD warranty services and is available only for qualified users that have made a valid purchase of a 2nd Gen Ryzen™ Desktop Processor and are affected by this specific boot up issue.
This service is free of charge with the following condition:
Once you have submitted your claim for a processor loan boot kit, AMD will require pictures of your 2nd Gen Ryzen™ Desktop Processor and AMD Socket AM4 300-series motherboard, that clearly shows the model numbers and unique serial numbers as well as a copy of the purchase invoice(s) to authorize the request. Additionally, a summary or copy of communication with the motherboard manufacturer is requested to indicate why support from the Original Design Manufacturer (ODM) is not suitable.
Once the RMA request is approved, the AMD AM4 A-series processor will be sent with pre-paid return shipping. The processor is provided as a temporary loan to you for the sole purpose of updating the BIOS and must be returned within 10 business days of receipt. It is not necessary to return the provided thermal solution.
To obtain the boot kit, please carefully follow these instructions:
Go to the AMD online warranty claims page:
https://support.amd.com/en-us/warranty/rma Fill in your full contact and product details (2nd Gen Ryzen Processor OPN number and serial number)
In the Problem Description field enter "Boot kit Required" (without quotes)
NOTE: The product must be a valid AMD processor with authentic OPN and serial number.
Once the claim is received and approved, confirmation details and boot kit instructions will be provided in a follow up email.