<human>: So my ex (28M) and i (27F) broke up 3 months ago (1 year relationship). He lied to me. I discovered we was seeing someone else. He is still lying to me. 

Saying he is not in a relationship, pretending he cares about me and wants me back, he even have a Christmas gift for me and wants to see me. I replied that we broke up and shouldn't give gifts to each other and the only gift i want is the money he owes me.
Also wants to see me because he wants to go back home close to his family (i was ok but in 2 years) and wanted to talk to me in person about that during a dinner.

He said he spent few days out of town to visit a friend and was alone for Christmas because he had to travel for work. 

I know who is the girl and i've checked on facebook. He is not at all travelling for work but spending holidays with her family and there is some couple pictures of them.

If i was the girlfriend i would like to know what my boyfriend is doing behind my back.

I don't want him back. I just want he leaves me alone. For now, i am nice with him because he owes me money. He is supposed to give me the last payment this week. So after he paid me, should i tell him i know he is lying? And/her his gf too?

Condense the text into a few key points
<bot>: ex bf lying to me (ex gf) saying he is not in relationship and wants me back. Should i tell his gf?