I need girls advice!

Hello my beautiful ladies! Hope you've all had a lovely weekend! Right situation is, boyfriend of a year and a half, absolutely utterly in love with him! We both had been in relationships before we met but my boyfriend met this girl in school (he's now 21) and was absolutely bessotted by her up until a few months before he met me but she was never interested and we know how mean girls can be! 
I understood the fact her adored her and she didn't want him but once he actually asked me to be his girlfriend he never mentioned her! 
To cut a long story short we go out quite a bit and his friends come out, my friends come out. But Beth, who I get along with really well! I've just found out is the girl that he was madly in love with. Now Beth has been coming with us to things and we give her lifts to places just because I thought she was a really close friend of my boyfriends. Until his best friend George said to me "I can't believe you actually get on with that bitch after what she did to Simon"....
I don't really know what to think at this moment in time, any support or advice would be much appreciated! 
Thanks in advance girlies xx