10:40 AM PT -- Patrick didn't do Miley dirty ... he says the chick is just a friend. Moments after we posted the pics, he tweeted, "Omg. It's one of my best friends girlfriend" -- and added, he would never step out on his GF. Patrick Schwarzenegger might be taking a wrecking ball to his relationship with Miley Cyrus ... as he got very close with some OTHER chick in Cabo this weekend.
It's also possible this girl is just a friend ... though "friends" don't usually do body shots off one another.
Then again, when Miley is your girlfriend, something like this is probably no big deal.
Patrick seemed pretty chill about everything in this Instagram shot.