Disclaimer: I Do Not Own Harry Potter.
She's curled up asleep on the old mattress in the corner of the cell, his cell door is unlocked and he could run. Something he'd wanted to for months, but now he has the opportunity he doesn't want too.
No, that's wrong he does want to run but he knows that she'll be blamed for letting him escape. Even if she lies about how he was able to escape in the first place she'll still lose face.
So he doesn't run, he gently pushes her further to the wall to make room for him on the mattress and lays down beside her.