Confused and depressed

hi all first time poster! I was just looking fwor some advice... im a male and going out with a nice girl and im happy enough in our relationship.... but when she was 19 she had her had sex (for the first time) with a man 15 years older he was 34 at the time , they met up a good few times after that secretly as she said... that she didnt want her parents finding out etc.
we have told another about our past but this keeps playing on my mind cause ya mans from around and does be telling mutual friends about this young I r##e etc. he also sometimes works in her workplace ! ive also noticed she has blocked him on Facebook, but yet she has old messages in her phone from him still... lookit il b honest I no the past is the past but it is effecting me alot and I dont want it to ruin us ... she tells me everything and is a great girl but I just want advice as to how to talk this through etc... thanks in advance and sorry for the long post