He has a girlfriend

About 5 years ago I met this guy in a college psychology class and we hit it off. We dated for 2 years then called it quits. We have always remained friends but started becoming romantic again about 8 months ago. He lives an hour away so we don't see each other super often. I noticed over time weird things he would do suggesting maybe he has a girlfriend. For example he has 2 phones. He told me one is for work but I always secretly questioned that. He also would always have to step outside a number of times to call someone or take a call. Last night he finally invited me over to his new house that he got several months ago and right away things were off. I noticed pink girls shoes by the front door and when I asked whos shoes they were he said his roommates. He never mentioned a roommate til that moment. He also told me he doesn't have pets but when I arrived I was greeted by a dog and later on a cat. I had to pee a little after arriving and he insisted I use the basement bathroom. Once I was in there I knew for sure he was full of shit because I found a note by the sink saying "I love you! Have a great day at work" and it was signed by a woman. I finally confronted him and he came clean. He has had a gf for a year! We have had sex and everything. I can't believe I didn't run when I saw the warning signs. I am done with him and all that but i'm still in shock. Should I tell the girl? What would you do?