Well, if the key is Bb major, then Cb should be bII in that key, pretty much no matter what its purpose is. Unless there's some modulation or weird temporary tonicization going on, Cb is correct - there should be only one "B" note in a key, and Bb already fills that spot. Guitar players may gripe about it, but it's better form to write it correctly as Cb. If the key were E major, you'd obviously write it as F, not E#, so here in Bb, write it as Cb.

Of course, nothing's stopping you from leaving some editorial note that Cb = B somewhere obvious in the lead sheet. If you do that, you can likely preempt any whining!