Hooked up with buddys girl...do I have to tell him?

[hoyasaxa]

Friday night, Im out chillin with some friends in my nextdoor neighbors backyard. its around 3 am. I roll home to pick up a chair, and theres a girl literally waiting for me on my couch.
She spends the night.
We spent most of yesterday together too. She spent the night again.

The problem is, my buddy already hooked up with this girl a couple weeks ago. I actually ran into her on a walk of shame from his place. Im not sure if he likes her or if it was a one time thing.

Do I have to tell him about this? Do i owe him an apology? Or is this just my business?