Am I overreacting?Boyfriend of one year msging (yes an old friend) a female friend inappropriately.Invites etc

My boyfriend of a little over a year has a female friend whom he has known for 5 years. 5years ago they would 'bump' into each other while out and 'hook up' but always end up going home alone. (He has told me this)
I do not trust this girl as I've heard a few stories myself, and unfortunately have begun to distrust him.
After a drunken night out I looked in his phone, found msgs from throughout the week to and from this girl.
Inviations to hang out, to stayover, to go for a trip to view a car with him, on a night which he said he wanted a boys night.

Obivously I confronted him, he still claims all innocence. Shes a good friend, she bought a new car, wanted to hang out, incase she wanted a few drinks, i wanted you to look at cars alone and I wanted company on the drive.

He acknowledged he was wrong to invite her out on the 'boys night' after dissing me.

She has since msgd atleast once to my knwledge (asking how th night went) and he claims he has not reponded.

I do not want to be the evil girlfriend.

I have asked him if he wants me or her and he says me, otherwise he would not be here still.

Doesnt help that his parents LOVE her and HATE me!

They have crossed a certain line, but am I overreacting?