It takes a few minutes to get an NPI if you do the online application. One provider cannot use another provider's NPI for billing simply because they don't have one. If they are doing an internship, or some other situation where they require supervision, then they can bill under the supervisor, but I'm not sure that's the case here. Does the provider require supervision? Do they have a limited license? If it's just because they don't have an individual NPI I would look into getting one ASAP. Then they will have to be credentialed with the insurances.