Have been seeing a girl from work for a while now but she tends to flirt with another guy. She says she is not interested in him that way she is just a friendly girl. Is she telling the truth?

We strangers online can't possibly know what she is thinking.
Do people fib, lie outright, and bend the truth in relationships? Of course.
I would believe her until you have proof of something happening past flirting.
A healthy relationship is often based on some harmless little freedoms.
That way you don't get bogged down too much with each other. Try a little more interaction with other women in your workplace. They know you have a girlfriend, so it's easier to be friendly.