Yeah, if Derek sees him outside Sakura's work, he's just going to tell Sakura that there is boy outside looking at her and is going to ask her if he is one of her friends. Or, he will just ask Li if he is going in.

Derek is slightly protective of Sakura because he feels she has no sense of safety at it alarmed him the first time he met her.

Eventually he will be, "Li, that girl is completely clueless, you need to keep an eye on her."

Right now Derek doesn't believe that magic is real. Hopefully that will change soon. But right now he is in hard denial that magic exists. He has his baby magic back, but hasn't fully realized this yet.