Hertz.com does detect locations based on IP address.

Note that if your friends are from Hong Kong they may get a rate with lower or no airport surcharges, free LDW and LIS, etc., which might mean that they do better than someone in the US. This is more likely to be the case at an airport rather than at an HLE.

Most CDPs will work wherever the renter is from (e.g., frequent flyer program, credit card rates) but others (particularly corporate rates) might book into a particular rental class that won't be available to international renters.

Which CDP were you using? Do they have any hotel, frequent flyer or credit cards that they could use to get a discount?