A guy best friend

I have started dating a girl from almost 6 months, but later on it's found that she has another male best friend and she know him from previous 5 years. Although they haven't been physical but after stalking her social media I have found that she is very interested in him. She also do flirtatious comments on his posts to get the attention. She is also in contact with him on WhatsApp from previous 5 years and she also shared her personal things with him as she told me. I am suspicious BECAUSE I know him and He is an alcoholic and also has many physical relationships with other girls. He is a great manipulator of words and my girlfriend usually talks to him at night when you know the hormones are on its peak 😊 and words manipulation can lead girls to any direction (intimate talks/sexting). So is that possible that she wasn't trapped by this manipulator in intimate talks/sexting?? Girls have more good insight on this issue because they better know how the late night talks works? It's very important for me to decide now because extramarital affairs suck individuals a lot? I have seen that... Thank you