She's about to tell him that if he tries to do anything in his state that might result in her getting hurt, then it is, by definition, dangerous and stupid. But he seems insistent on going on with it, whatever it is, and so she decides that he could have been really stupid and try to do whatever this is on his own instead of with her there.

So while she really doesn't want to, Hermione carefully maneuvers herself slightly behind Jamie, still holding onto him securely. Glancing at his shard hand with a concerned frown on her face, she murmurs, "Just promise me that you won't hurt yourself."