Your friend is being very picky. You are allowed to say realtor or real estate agent. Regardless of what word you use it all means the same thing, a person who sell houses. There is no law that say you can't.
Technically, 'realtor' refers only to people who are members of the National Association of Realtors. However, it is now commonly used for all those who sell or lease real estate.
I think it is just because real estate agent carries more formality with it. It is like calling a police officer cop. This really has to do with ego though because I don't see an issue.
The term "Realtor" is a trademark of the National Association of Realtors. Most real estate agents are members and can call themselves "Realtors," but not all.
