<human>: Can you provide a brief overview of the text?

My bf has been spending a lot of time helping his ex out recently as she recently moved back to town after a bad breakup and they share a son.  I was totally comfortable with that until his friend innocently asked me about a movie we supposedly went to see the night before & how he had needed help with a flat but my bf didn't return any of the texts.  He had told me that he was with the friend who was asking.   That got me suspicious so the next night, I rode by her house on my way home from a dinner for work pretty late & his car was there.   I have sat on this information for awhile & didnt confront him until last week when he lied to me again about where he was the night before.   I asked him out right if he was back with her & he denied it.  He then turned it around on me & said that I am insecure(which I admit I probably am because he is lying to me) so I cowered in confronting him about his lying.   He then wanted to know why I thought this and who told me.
<bot>: I dont want to get his friend involved and I don't want to tell him I drove by cause he will turn it around on me & say I am crazy--how would you handle telling him how I know?