My gf is a very caring person who thinks that she can help anybody and I love that about her- I really do. She has a heart of gold and believes in the good in people. I for my part am very cautious with my trust because of bad experiences. 
She also has this colleague who always helps her, wants her to go to the gym with him because he is very athletic and my gf likes to work out..long story short I have always had a bad feeling about this guy and have often told her so.
She on the other hand thinks of him just as a work buddy and tried to calm me by showing me bits of their chats. This didn't exactly calm me, but made my concerns stronger because of the frequency they talk to each other. 
This week he was sick because of a heart issue and she wanted him to tell her how his test results are as soon as possible because "she is curious what it could be". 
Her reasons for being so faithful in this guy's honesty is on the one hand because he told her he just had gotten out of a bad relationship and doesn't want another girlfriend at the moment and an additional reason is that she doesn't have many friends and wanted to make some new in her new job. 
When we argumented she said that when I am allowed to have female friends why isn't she allowed to have some guys as friends. I then told her that she knows all of my friends and all of them are in a relationship, I on the other hand have never met her colleague. 
At the end some facts: 
We are together for 4 years 
We moved in together a half year ago

So I want to know: AITA for wanting her to not have contact with this guy outside of work?