Rebeljp18

I have a question I have a girlfriend who has been friends with a guy for 15 years. After she got divorced he also was going thru a divorce. After talking they had sex a few times. Now that we are together she and him are still friends and contiune to talk and hang out together. Is this something that can be done cause I know it would be hard for me to do something like this. "We are just friends whats done is done and we both know that".