I suspect that my girlfriend's coworker is flirting with her.
Are you worried about your relationship?
Yes. I think my girlfriend loves him. Maybe I should let her go.
You shouldn't assume the worst of people when you don't actually know it's true. It's very possible that they aren't flirting and she doesn't love him.
You are wrong. You don't know what my girlfriend thinks.
It sounds like you and your girlfriend need to sit down and have a serious relationship conversation. If you are ready to give up your relationship so easily when you have no proof doesn't say much for your feelings. Talk to her!