Remember the part where jersey gets out of bed to find the keyboard set up by the guy, (I'm pretty vague on details), he persuades her to play him her song, and magically the keyboard already knows the tempo to her song! oops
She was playing on the keyboard before he got out of bed-she could have programmed the keyboard. She already knew what she wanted the song to sound like.
It's not her keyboard, because Kevin says when he brings her down there that he borrowed it from the club, and to go easy on it. The reason he borrowed it for her was because he wanted her to have a place to practice, and it's obvious she couldn't practice in her apartment because her neighbors would freak out (cause the walls are too thin...apartments made so cheap these days...:O)
Remember when they were coming into HIS apartment, he says "Be gentle, I borrowed it from the club." Huh??? But she did already program her "beat" into the keyboard.