Great question! I’d be happy to share what insight I can into these type of order alerts.
The first thing I recommend doing is opening the customer profile in your admin and use the “Request customer data” under the More Actions tab. The CSV file you receive will include a detailed breakdown of payment attempts and order risk analysis. This is your customer’s data, so you can also forward these files to them for their own information. You may see more information in those CSV files that helps to answer why they are being flagged as a higher risk.
Some things to note as well, if the customer is using a VPN or public internet connection (coffee shop, library, etc) then the IP address they are purchasing from could be the issue. They should attempt checkout on a private, no VPN, internet connection.
Shopify uses a wide range of tooling to assess order risk and, more often than not, high risk orders almost always result in a chargeback. If you wish to continue processing this customer’s order I would be upfront with them about the order alerts you are seeing and ask them for as much evidence and proof of purchase as possible to cover your business. Asking for images of current ID, email/text confirmation of the order placement, etc.