It's important to note that:
* Origin is not destiny: Just because someone has a Cambodian last name doesn't mean they are Cambodian. They could have heritage from other places, or be adopted.
* Last names can be common across regions: While Taing is more common in Cambodia, it may exist in other populations as well.
To know someone's nationality, it's best to ask them directly or, if researching someone, use other information available.